So a bunch of miscellaneous findings, mostly from trying to get yamanba to work in my feudal japan mod:
Making something multiple differnt types of "megabeast" (night creature hunter, semimegabeast, etc) will make that creature spawn in every possible category.
Turning people into vermin is REALLY WEIRD
Because legends mode will not track vermin, it will also not track people being turned into vermin. This includes people turned into vermin during normal gameplay, legends mode will just pretend they were their original species.
People turned vermin are not treated like normal vermin and instead are just treated as normal creatures. Basically you cannot Grab them with g in adv mode and they don't blink around.
I managed to crash the game by trying to throw a guy I turned into a snail with wrestling. However I can't reproduce this.
If creatures have multiple specialattack_interactions they tend to use them all at once during worldgen
specialattack_interaction interactions actually occur BEFORE the attack that inflicts them (important enough that I already added to the wiki). Also how I got around the weirdness with turning people into bugs, because they can't act weird if they're 100% guaranteed to be squished the instant after they turn into a bug!
worldgen battles seem to be decided mostly be size and material composition (of the armor, weapons, and creatures) that take part in them. Not sure about skills (aside from things like military tactics for group engagements), but if skills do matter 10 striking is not enough to make any noticeable difference. Notably, the properties of actual attacks used don't seem to matter that much. My yamanba, despite being insanely dangerous in actual gameplay, are such chumps in worldgen that they almost never manage to get names and only seem to """win""" most of their engagements because units are programmed to flee from megabeasts instead of fighting. They tend to die as soon as someone "confronts" them. The thing this say the most to me is that velocity of attacks probably isn't considered, since the way I got my mountain hags to break the skin with their nails to inflict their curses was to make them scratch really really really fast.
Adventurers cannot be made to eat bugs, even if you give them the gobble_vermin tag. Sad
Megabeasts seem to be able to become the enemy of something without interacting with them. I had multiple creatures with literal screens worth of enemies who had done nothing aside from spawn in a mountain somewhere.