So the way civs are placed is, the game takes the seed and selects an entity as a starting point, then goes down the list of entities from there (not exactly sure how the list is ordered, but its probably as they appear in the raw files, eg starting with the first entity in the first entity file alphabetically and then going down to the last entity in the last file.) When/if it gets to the end of the list, it will start over from the beginning, and continue going through that loop until it's filled up the number of entities the world gen parameters call for. If it places the desired number of civs but hasnt placed one that's playable, then worldgen will restart with a new world and attempt it again, unless advanced parameters were used to tell it to skip playable civ checks (which only counts fortress-mode, not adventure mode playable).
So if youve got a list of 150 civs, and the game starts on the 77th, places a total of 50, then your world will include civs 77 through 126. The rest wont appear in the world because it stops when it hits the desired number. If none of them are playable in fortress mode, it will skip, so you should make maybe 1 in 10 civs playable at least and spread them evenly throughout the raws, once every ten civs, so theres a good chance that even on tiny worlds it will place at least one of them and wont skip worlds because it didnt place a playable civ.
If you want to guarantee, say, every world will include the vanilla civs, you'd have to include a duplicate of them every five or six civs in the raws, so do five animal man civs then five duplicates of the vanilla civs. That would guarantee half the world would be the vanilla races and half animal people.
Alternatively, you can reduce your number of animal man civs by merging species into the same civ by some category. It will still only pick one species for the civ (cant have multi-species civs right now) but it will reduce the total length of the list of civs. You could do, say, one tiny bug person civ, one rodent civ, one arctic civ, one temperate herbivore civ, one temperate predator civ, one temperate omnivore civ, one tropical herbivore civ, one tropical predator civ, one tropical omnivore civ. That would give a list of 9 additional civs, and the game would rendomly select which species represents that civ during worldgen. You could add each species to one or more of them, then. But, the list of civs including vanilla would only be 15 (assuming the subterrranean animal people civ is included), which is a reasonable length of civs for the average world to include most or all of them plus a few, and large worlds with high numbers of civs to go through the list two or three times.