There's a few things we can do, not sure if it's what you're looking for but, it's what's possible currently with the limitations of DF. Lets take a look. The raws are found in 'entity_default.txt' which can be found within the df/raw/objects directory.
Their civilization raws:
[ENTITY:SUBTERRANEAN_ANIMAL_PEOPLES]
[LAYER_LINKED]
[CREATURE:AMPHIBIAN_MAN]
[CREATURE:REPTILE_MAN]
[CREATURE:SERPENT_MAN]
[CREATURE:RODENT MAN]
[CREATURE:BAT_MAN]
[CREATURE:ANT_MAN]
[CREATURE:OLM_MAN]
[CREATURE:CAVE_SWALLOW_MAN]
[CREATURE:CAVE_FISH_MAN]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_BLOWGUN]
[AMMO:ITEM_AMMO_BLOWDARTS]
[SHIELD:ITEM_SHIELD_SHIELD]
[SHIELD:ITEM_SHIELD_BUCKLER]
[WOOD_WEAPONS]
[WOOD_ARMOR] shields
[USE_ANY_PET_RACE]
[INDOOR_WOOD]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[EQUIPMENT_IMPROVEMENTS]
[FRIENDLY_COLOR:1:0:1]
no site or biome or attack info for now
[MAX_STARTING_CIV_NUMBER:100] all irrelevant right now
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:WOODCRAFTER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_REACTION:TAN_A_HIDE]
*** ethics copied from kobolds for now
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_EXILE]
[ETHIC:KILL_NEUTRAL:REQUIRED]
[ETHIC:KILL_ENEMY:REQUIRED]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:NOT_APPLICABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:NOT_APPLICABLE]
[ETHIC:LYING:NOT_APPLICABLE]
[ETHIC:VANDALISM:NOT_APPLICABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_KILL:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
*** later
[VALUE:LAW:0]
[VALUE:LOYALTY:0]
[VALUE:FAMILY:0]
[VALUE:FRIENDSHIP:0]
[VALUE:POWER:0]
[VALUE:TRUTH:0]
[VALUE:CUNNING:0]
[VALUE:ELOQUENCE:0]
[VALUE:FAIRNESS:0]
[VALUE:DECORUM:0]
[VALUE:TRADITION:0]
[VALUE:ARTWORK:0]
[VALUE:COOPERATION:0]
[VALUE:INDEPENDENCE:0]
[VALUE:STOICISM:0]
[VALUE:KNOWLEDGE:0]
[VALUE:INTROSPECTION:0]
[VALUE:SELF_CONTROL:0]
[VALUE:TRANQUILITY:0]
[VALUE:HARMONY:0]
[VALUE:MERRIMENT:0]
[VALUE:CRAFTSMANSHIP:0]
[VALUE:MARTIAL_PROWESS:0]
[VALUE:SKILL:0]
[VALUE:HARD_WORK:0]
[VALUE:SACRIFICE:0]
[VALUE:COMPETITION:0]
[VALUE:PERSEVERANCE:0]
[VALUE:LEISURE_TIME:0]
[VALUE:COMMERCE:0]
[VALUE:ROMANCE:0]
[VALUE:NATURE:0]
[VALUE:PEACE:0]
[AMBUSHER]
[STONE_SHAPE:OVAL_CABOCHON]
[GEM_SHAPE:OVAL_CABOCHON]
When you look through these, you'd expect this code to be the culprit:
[MAX_STARTING_CIV_NUMBER:100]
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
However, it appears that this code does absolutely nothing currently. Which means we need to keep looking. Upon further inspection you'll notice, no biome information? Where will they settle? We can copy the biome information from humans, that is, if you're looking to find civilizations of giant humanoid rats hanging out. The new raws would look like this:
[ENTITY:SUBTERRANEAN_ANIMAL_PEOPLES]
[LAYER_LINKED]
[CREATURE:AMPHIBIAN_MAN]
[CREATURE:REPTILE_MAN]
[CREATURE:SERPENT_MAN]
[CREATURE:RODENT MAN]
[CREATURE:BAT_MAN]
[CREATURE:ANT_MAN]
[CREATURE:OLM_MAN]
[CREATURE:CAVE_SWALLOW_MAN]
[CREATURE:CAVE_FISH_MAN]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_BLOWGUN]
[AMMO:ITEM_AMMO_BLOWDARTS]
[SHIELD:ITEM_SHIELD_SHIELD]
[SHIELD:ITEM_SHIELD_BUCKLER]
[WOOD_WEAPONS]
[WOOD_ARMOR] shields
[USE_ANY_PET_RACE]
[INDOOR_WOOD]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[EQUIPMENT_IMPROVEMENTS]
[FRIENDLY_COLOR:1:0:1]
-----! This is where the problem is. There's nowhere for them to settle, which means they're not gonna appear anywhere except underground, that will be talked about later though ----!no site or biome or attack info for now
Human's site/biome info:
[DEFAULT_SITE_TYPE:CITY]
[LIKES_SITE:CITY]
[TOLERATES_SITE:CITY]
[TOLERATES_SITE:TREE_CITY]
[TOLERATES_SITE:CAVE_DETAILED]
[START_BIOME:ANY_GRASSLAND]
[START_BIOME:ANY_SAVANNA]
[START_BIOME:ANY_SHRUBLAND]
[BIOME_SUPPORT:ANY_WETLAND:1]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:ANY_OCEAN:12]
[BIOME_SUPPORT:ANY_LAKE:3]
[BIOME_SUPPORT:ANY_GRASSLAND:3]
[BIOME_SUPPORT:ANY_SAVANNA:2]
[BIOME_SUPPORT:ANY_SHRUBLAND:2]
[BIOME_SUPPORT:ANY_RIVER:4]
[MAX_STARTING_CIV_NUMBER:100] all irrelevant right now
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:WOODCRAFTER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_REACTION:TAN_A_HIDE]
*** ethics copied from kobolds for now
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_EXILE]
[ETHIC:KILL_NEUTRAL:REQUIRED]
[ETHIC:KILL_ENEMY:REQUIRED]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:NOT_APPLICABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:NOT_APPLICABLE]
[ETHIC:LYING:NOT_APPLICABLE]
[ETHIC:VANDALISM:NOT_APPLICABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_KILL:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
*** later
[VALUE:LAW:0]
[VALUE:LOYALTY:0]
[VALUE:FAMILY:0]
[VALUE:FRIENDSHIP:0]
[VALUE:POWER:0]
[VALUE:TRUTH:0]
[VALUE:CUNNING:0]
[VALUE:ELOQUENCE:0]
[VALUE:FAIRNESS:0]
[VALUE:DECORUM:0]
[VALUE:TRADITION:0]
[VALUE:ARTWORK:0]
[VALUE:COOPERATION:0]
[VALUE:INDEPENDENCE:0]
[VALUE:STOICISM:0]
[VALUE:KNOWLEDGE:0]
[VALUE:INTROSPECTION:0]
[VALUE:SELF_CONTROL:0]
[VALUE:TRANQUILITY:0]
[VALUE:HARMONY:0]
[VALUE:MERRIMENT:0]
[VALUE:CRAFTSMANSHIP:0]
[VALUE:MARTIAL_PROWESS:0]
[VALUE:SKILL:0]
[VALUE:HARD_WORK:0]
[VALUE:SACRIFICE:0]
[VALUE:COMPETITION:0]
[VALUE:PERSEVERANCE:0]
[VALUE:LEISURE_TIME:0]
[VALUE:COMMERCE:0]
[VALUE:ROMANCE:0]
[VALUE:NATURE:0]
[VALUE:PEACE:0]
[AMBUSHER]
[STONE_SHAPE:OVAL_CABOCHON]
[GEM_SHAPE:OVAL_CABOCHON]
The above code still has a few things we need to iron out. We need to remove the token [LAYER_LINKED] because it makes the civilization a hidden underground one. We would keep it that way but, we can't influence generation any other way, by the raws atleast. Next, I'm not sure about you but, I'd rather not have them build giant cities. Not very rat-men-like. If that's what you fancy, you can remove the [LAYER_LINKED] token and use these raws. However I think CAVE_DETAILED would be a much better site choice. I mean could you imagine a huge cave detailed with tons of engravings about the struggles of alligator-man-life? Lets take a look and make some changes to that section.
Human's site/biome info:
[DEFAULT_SITE_TYPE:CITY]
[LIKES_SITE:CITY]
[TOLERATES_SITE:CITY]
[TOLERATES_SITE:TREE_CITY]
[TOLERATES_SITE:CAVE_DETAILED]
[START_BIOME:ANY_GRASSLAND]
[START_BIOME:ANY_SAVANNA]
[START_BIOME:ANY_SHRUBLAND]
[BIOME_SUPPORT:ANY_WETLAND:1]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:ANY_OCEAN:12]
[BIOME_SUPPORT:ANY_LAKE:3]
[BIOME_SUPPORT:ANY_GRASSLAND:3]
[BIOME_SUPPORT:ANY_SAVANNA:2]
[BIOME_SUPPORT:ANY_SHRUBLAND:2]
[BIOME_SUPPORT:ANY_RIVER:4]
New site/biome info:
[DEFAULT_SITE_TYPE:CAVE_DETAILED]
[LIKES_SITE:CAVE_DETAILED]
[TOLERATES_SITE:CAVE]
[START_BIOME:ANY_GRASSLAND]
[START_BIOME:ANY_SAVANNA]
[START_BIOME:ANY_SHRUBLAND]
[START_BIOME:MOUNTAIN]
[BIOME_SUPPORT:ANY_WETLAND:1]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:MOUNTAIN:12]
[BIOME_SUPPORT:ANY_GRASSLAND:3]
[BIOME_SUPPORT:ANY_SAVANNA:2]
[BIOME_SUPPORT:ANY_SHRUBLAND:2]
What did we do? We changed the default site they build and the site they like to CAVE_DETAILED, and made them like CAVE. We then added a start biome (where the civilizations are 'born) to mountains. Then we removed the lakes/ocean/rivers from the biome support because, personally, I don't think that would be a good choice for caves. Lastly we are going to make sure they have biome support for mountains, with the rate of '12' so they're much more likely to go to the mountains than anywhere else. Great! Lets toss it all together and see our finished raw for the civilization!
[ENTITY:SUBTERRANEAN_ANIMAL_PEOPLES]
[CREATURE:AMPHIBIAN_MAN]
[CREATURE:REPTILE_MAN]
[CREATURE:SERPENT_MAN]
[CREATURE:RODENT MAN]
[CREATURE:BAT_MAN]
[CREATURE:ANT_MAN]
[CREATURE:OLM_MAN]
[CREATURE:CAVE_SWALLOW_MAN]
[CREATURE:CAVE_FISH_MAN]
[WEAPON:ITEM_WEAPON_SPEAR]
[WEAPON:ITEM_WEAPON_BLOWGUN]
[AMMO:ITEM_AMMO_BLOWDARTS]
[SHIELD:ITEM_SHIELD_SHIELD]
[SHIELD:ITEM_SHIELD_BUCKLER]
[WOOD_WEAPONS]
[WOOD_ARMOR] shields
[USE_ANY_PET_RACE]
[INDOOR_WOOD]
[USE_CAVE_ANIMALS]
[USE_ANIMAL_PRODUCTS]
[EQUIPMENT_IMPROVEMENTS]
[FRIENDLY_COLOR:1:0:1]
New site/biome info:
[DEFAULT_SITE_TYPE:CAVE_DETAILED]
[LIKES_SITE:CAVE_DETAILED]
[TOLERATES_SITE:CAVE]
[START_BIOME:ANY_GRASSLAND]
[START_BIOME:ANY_SAVANNA]
[START_BIOME:ANY_SHRUBLAND]
[START_BIOME:MOUNTAIN]
[BIOME_SUPPORT:ANY_WETLAND:1]
[BIOME_SUPPORT:ANY_DESERT:1]
[BIOME_SUPPORT:ANY_FOREST:2]
[BIOME_SUPPORT:MOUNTAIN:12]
[BIOME_SUPPORT:ANY_GRASSLAND:3]
[BIOME_SUPPORT:ANY_SAVANNA:2]
[BIOME_SUPPORT:ANY_SHRUBLAND:2]
[MAX_STARTING_CIV_NUMBER:100] all irrelevant right now
[MAX_POP_NUMBER:10000]
[MAX_SITE_POP_NUMBER:120]
[PERMITTED_JOB:CARPENTER]
[PERMITTED_JOB:WOODCUTTER]
[PERMITTED_JOB:ANIMAL_CARETAKER]
[PERMITTED_JOB:ANIMAL_TRAINER]
[PERMITTED_JOB:HUNTER]
[PERMITTED_JOB:WOODCRAFTER]
[PERMITTED_JOB:LEATHERWORKER]
[PERMITTED_JOB:BONE_CARVER]
[PERMITTED_JOB:FISHERMAN]
[PERMITTED_JOB:FISH_CLEANER]
[PERMITTED_JOB:COOK]
[PERMITTED_JOB:BUTCHER]
[PERMITTED_JOB:TANNER]
[PERMITTED_REACTION:TAN_A_HIDE]
*** ethics copied from kobolds for now
[ETHIC:KILL_ENTITY_MEMBER:PUNISH_EXILE]
[ETHIC:KILL_NEUTRAL:REQUIRED]
[ETHIC:KILL_ENEMY:REQUIRED]
[ETHIC:KILL_ANIMAL:ACCEPTABLE]
[ETHIC:KILL_PLANT:ACCEPTABLE]
[ETHIC:TORTURE_AS_EXAMPLE:UNTHINKABLE]
[ETHIC:TORTURE_FOR_INFORMATION:NOT_APPLICABLE]
[ETHIC:TORTURE_FOR_FUN:ACCEPTABLE]
[ETHIC:TORTURE_ANIMALS:UNTHINKABLE]
[ETHIC:TREASON:UNTHINKABLE]
[ETHIC:OATH_BREAKING:NOT_APPLICABLE]
[ETHIC:LYING:NOT_APPLICABLE]
[ETHIC:VANDALISM:NOT_APPLICABLE]
[ETHIC:TRESPASSING:NOT_APPLICABLE]
[ETHIC:THEFT:NOT_APPLICABLE]
[ETHIC:ASSAULT:PERSONAL_MATTER]
[ETHIC:SLAVERY:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_OTHER:UNTHINKABLE]
[ETHIC:EAT_SAPIENT_KILL:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAME_RACE:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_SAPIENT:UNTHINKABLE]
[ETHIC:MAKE_TROPHY_ANIMAL:UNTHINKABLE]
*** later
[VALUE:LAW:0]
[VALUE:LOYALTY:0]
[VALUE:FAMILY:0]
[VALUE:FRIENDSHIP:0]
[VALUE:POWER:0]
[VALUE:TRUTH:0]
[VALUE:CUNNING:0]
[VALUE:ELOQUENCE:0]
[VALUE:FAIRNESS:0]
[VALUE:DECORUM:0]
[VALUE:TRADITION:0]
[VALUE:ARTWORK:0]
[VALUE:COOPERATION:0]
[VALUE:INDEPENDENCE:0]
[VALUE:STOICISM:0]
[VALUE:KNOWLEDGE:0]
[VALUE:INTROSPECTION:0]
[VALUE:SELF_CONTROL:0]
[VALUE:TRANQUILITY:0]
[VALUE:HARMONY:0]
[VALUE:MERRIMENT:0]
[VALUE:CRAFTSMANSHIP:0]
[VALUE:MARTIAL_PROWESS:0]
[VALUE:SKILL:0]
[VALUE:HARD_WORK:0]
[VALUE:SACRIFICE:0]
[VALUE:COMPETITION:0]
[VALUE:PERSEVERANCE:0]
[VALUE:LEISURE_TIME:0]
[VALUE:COMMERCE:0]
[VALUE:ROMANCE:0]
[VALUE:NATURE:0]
[VALUE:PEACE:0]
[AMBUSHER]
[STONE_SHAPE:OVAL_CABOCHON]
[GEM_SHAPE:OVAL_CABOCHON]
Copy the code and replace the existing one and you'll no longer find them in the caverns, you will, however, have them around the world. They might still get killed during world generation because of their lack of weapons, which you can change if you'd like. They also will ambush you, which should be fun, but they won't siege you. If you want that, I'll leave it up to you, it's not too hard though just go check the wiki for the entity tokens and add a specific token.
It is a bummer we can't just buff up how many are added, but maybe this will add extra !!FUN!! to the game
Have !!FUN!! And feel free to ask any questions I'll try to answer as soon as I can.