Try this:
[WORLD_GEN]
[TITLE:MESS2]
[CUSTOM_NAME:Fiora]
[DIM:65:65]
[EMBARK_POINTS:10000]
[END_YEAR:300]
[BEAST_END_YEAR:300:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:101:360:3200:3200]
[RAINFALL:0:100:3200:3200]
[TEMPERATURE:50:75:3200:3200]
[DRAINAGE:0:100:3200:3200]
[VOLCANISM:0:100:3200:3200]
[SAVAGERY:0:100:3200:3200]
[ELEVATION_FREQUENCY:2:90:0:0:0:10]
[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]
[MINERAL_SCARCITY:500]
[MEGABEAST_CAP:100]
[SEMIMEGABEAST_CAP:100]
[TITAN_NUMBER:33]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:52]
[NIGHT_TROLL_NUMBER:26]
[BOGEYMAN_NUMBER:26]
[VAMPIRE_NUMBER:26]
[WEREBEAST_NUMBER:26]
[SECRET_NUMBER:52]
[REGIONAL_INTERACTION_NUMBER:52]
[DISTURBANCE_INTERACTION_NUMBER:52]
[EVIL_CLOUD_NUMBER:26]
[EVIL_RAIN_NUMBER:26]
[GOOD_SQ_COUNTS:0:0:0]
[EVIL_SQ_COUNTS:0:0:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:0]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:100]
[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:250]
[RIVER_MINS:0:0]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:5000]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:60]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:60]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:100]
[CAVERN_LAYER_WATER_MIN:0]
[CAVERN_LAYER_WATER_MAX:100]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:15]
[LEVELS_ABOVE_LAYER_1:10]
[LEVELS_ABOVE_LAYER_2:5]
[LEVELS_ABOVE_LAYER_3:5]
[LEVELS_ABOVE_LAYER_4:5]
[LEVELS_ABOVE_LAYER_5:5]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:0]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:6]
[TOTAL_CIV_POPULATION:100]
[SITE_CAP:10]
[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]
A few things that I've observed (
in vanilla 34.11, ymmv)
Given you're playing fortress mode, a larger world doesn't really gain you anything. It just takes longer to load, save, and generate.
Generally speaking, there's no point in having a larger number of Civs beyond 6. That's how many entries are in entity_default, and duplicate civs of the same type won't visit any more often, annually. In other words, you can have 10,000 goblins, but you only get sieged once per season, because there is only ever one goblin "civilization" unless you modify the raws.
If you increase the site cap and pop cap values too high, you just end up with a ridiculously long worldgen which, 99% of the time, isn't fun.
A 'hodgepodge messy' world (if I understand what you mean by that) is primarily created by having sufficient Variance on Elevation, Rainfall, Temperature, Drainage, Savagery, and Volcanism. This creates hundreds or thousands of regions/biomes all mixed up together. This will lead to considerably more un-flat embarks, as a consequence.
Dwarves only need mountainous biome that is non-savage. If you have minimum savagery at 100, you will never be able to place the dwarven civilization, because all biomes are savage, and dwarves need non-savage.
The easiest way to ensure dwarves get their mountains is with the Elevation Weighted Range settings, ensuring that at least 10% of the world will be ~360 elevation. Contrary to popular belief, the initial mountainhome does not require peaks, just mountains.
Having a large number of megabeast caves, semi-megabeast caves, and mountainous caves has a few caveats. You can't have more than one titan per region, as far as I know. That is, if you set the worldgen parameter for titans to 1000, but only have 100 regions in your world, you will get at most 100 titans. I'm not sure if that applies for semi-megabeast and megabeasts as well, as they are, i believe, limited more by their caves.
I've not read any !!science!! related to semi-megabeast/megabeast placement, migration and concurrency. Forgotten beasts, i believe, are generated based on the number of underground regions.
The worldgen above generates without error/rejections, and all Variances are at 3200. It also generates a very large number of regions/biomes, which can be fun.
A typical population for that worldgen is:
Civilized World Population
241 Dwarves
440 Humans
435 Elves
10281 Goblins
235 Kobolds
Total: 21632
If any of that is unclear, let me know and I'll be happy to elaborate.