The problem of "powerful civs" and piles of megabeasts & titans has been hashed before, around page 8 of this thread iirc. They tend to be mutually exclusive, as you figured, though quick note: ocean gap can keep them out, and titans are randomly scattered on available land I guess.
As for FBs, larger words with more caverns generate more FBS (It's 1 per each underground region iirc).
I doubt titan attack limit would apply to non-player forts, but haven't tested. The other invasion limit settings don't, though.
With that in mind, here's few possibilities:
1) Tweaked variant of "civ & mega cagematch". One example:
Did manage to get major human town, but as you'd expect the original mentioned caveats apply. The elven major retreat is taken over by humans, and both them and dwarves have no sites.
89: Wishouthra, "Rhymingsculpted", town
Owner: The Society of Luster, humans
Parent Civ: The Savage Realm, humans
lady: Amsir Splatteredsmith, human
1221 goblins
1277 humans
32 elves
250 tick men
50 horses
67 chickens
94 blue peafowls
217 cats
162 dogs
8 guineafowls
110 turkeys
4 elf outcasts
26 goblin outcasts
28 human outcasts
3 dwarf outcasts
3: Kudosmunstu, "Growllies", dark fortress
Owner: The Sunken Seductions, goblins
Parent Civ: The Socketed Hate, goblins
lady: Xuspgas Threatspider, goblin
6615 goblins
1 human
2526 trolls
2526 ogres
2526 beak dogs
1 elf prisoner
Created in DF v0.43.05 linux 64-bit
[WORLD_GEN]
[TITLE:TweakCagematch4305]
[SEED:cIcSvyoJQfrDct4hRpGM]
[HISTORY_SEED:GgRpSL9IuA4KEJNo31xJ]
[NAME_SEED:UcKj61EJgGRZkg8truzb]
[CREATURE_SEED:ugWjnjNeW8jqWpNQgEZx]
[DIM:17:17]
[EMBARK_POINTS:1503]
[END_YEAR:250]
[BEAST_END_YEAR:1000:-1]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:99:400:0:0]
[RAINFALL:0:100:0:0]
[TEMPERATURE:83:100:1000:1000]
[DRAINAGE:0:100:0:0]
[VOLCANISM:0:100:0:0]
[SAVAGERY:0:100:0:0]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:1]
[VOLCANISM_FREQUENCY:1:1:1:1:1:1]
[POLE:NONE]
[MINERAL_SCARCITY:500]
[MEGABEAST_CAP:40]
[SEMIMEGABEAST_CAP:0]
[TITAN_NUMBER:30]
[TITAN_ATTACK_TRIGGER:7:0:0]
[DEMON_NUMBER:2]
[NIGHT_TROLL_NUMBER:0]
[BOGEYMAN_NUMBER:0]
[VAMPIRE_NUMBER:1000]
[WEREBEAST_NUMBER:1000]
[SECRET_NUMBER:1000]
[REGIONAL_INTERACTION_NUMBER:2]
[DISTURBANCE_INTERACTION_NUMBER:20]
[EVIL_CLOUD_NUMBER:2]
[EVIL_RAIN_NUMBER:0]
[GENERATE_DIVINE_MATERIALS:1]
[GOOD_SQ_COUNTS:0:70:0]
[EVIL_SQ_COUNTS:0:30:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:20]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:0]
[SUBREGION_MAX:32000]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:60]
[CAVERN_LAYER_OPENNESS_MAX:90]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:10]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:20]
[CAVERN_LAYER_WATER_MIN:51]
[CAVERN_LAYER_WATER_MAX:52]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:10]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:1]
[CAVE_MAX_SIZE:1]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:0]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:50]
[TOTAL_CIV_POPULATION:13000]
[SITE_CAP:-1]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
[PS_EL:299:330:330:330:330:330:330:330:330:330:330:330:330:330:330:330:330]
[PS_EL:299:299:299:299:299:299:299:299:299:299:299:299:299:299:299:299:299]
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34 titans and FBs, at least some vamps...But as for caveats?
Can remove the seeds and adjust civ counts or even demon counts - On one of the initial 200-civilizaton generations, I saw an elven forest retreat with 800 sentients and about 50k giant animals, though the reasons you might do this could also be wanting more surviving dwarves or dozen+ towers.
If you do adjust demon number as well as quadrupling civ count you might want to replace non-evil shrublands with marshes to prevent goblins from settling on human territories.
Can pick an embark place/site and then paint to make sure goblins can reach it, but not other sites. Granted, I don't expect this to make dwarves do better against megabeasts.
2) Perhaps a monster island with 4 civs instead of 1? Sure, they usually get wrecked, but not always, and gen doesn't take long.
3) 3 islands with human, elven and goblin civs. If you don't need major sites, then they can be 1-tile, though otherwise need to be at least 6 tiles for humans and elves. Then embark on an area where those three and mainland meet, so that you can have all neighbours. (This geography might need fair bit of regenerating.)
For the rest of the world, populate with megabeasts and dwarves somewhere. If you have less than 1/7 megabeasts per world tile it is probable that the foreign sites will be fine.
Oceans don't mesh well with visitors though iirc.