Well now they are shooting their own body parts disguised as bullets. This just causes massive bruising and no actual bullet effects on impact...
Well I need to finish making them have metal skin, but isn't there a way to specify what material or item you want them to attack with?
They're not shooting body parts, just like a GCS doesn't inject its body parts. I suspect that your bullet material might not be strong enough, thanks to the existing material problems (partially because cloth appears to be too good at blocking damage). A solid glob attack with INORGANIC:IRON rather than LOCAL_CREATURE_MAT:BULLET works well enough against an unarmored character, bruises a character clad in rope reed, and is rather ineffectual against characters clad in iron.
Slade on the other hand, can be deadly even if blocked by cloth. Try increasing the density of the bullet material.
Worldgen still doesn't place some creatures from the file, any idea why that might be? The creatures that aren't placed are primarily evil, which might be one reason since I genned a world that doesn't have large good/evil areas (to prevent the creation of good/evil mountain ranges and oceans), so could it be just that there isn't enough room for them? Some of the creatures include swamp trolls and mountain trolls which are basically identical to vanilla trolls except for different size and skin colour, so I don't think there's anything wrong with he code.
Yeah, if there aren't evil (enough?) regions of the needed biome, those creatures may end up not being placed, though sometimes, there are other factors at work. If they all worked when they were in a pre-existing file, this may simply be bad luck. Try increasing the evil parameters in worldgen for a test. The changed trolls, however, might have a different issue - if you changed their Biome away from subterranean chasm to a overground biome, you also need to either remove the underground depth tag, or, if you added the biome, change the depth tag to 0:3 .