So, I got one one my girls 'covered in bird-like feathers in many places', from overuse of the lab I think. However, I can't find a place in-game to remove them. I could try and edit the <save> file, but that doesn't seem optimal. Also, Is there an in-game place to see toxicity?

Ideas?

Cast mind reading on a slave and it tells you their toxicity, good idea to check it before having them drink any potions, or undergo any procedures.

Unfortunately there isn't a potion or lab option to change skin covering (feathers, scales, etc...) only skin color. The only way I can think to change skin covering is via the Mutation spell, where it's one of the many random attributes the spell can change. The odds are pretty low, but if you're really determined you can probably brute force the result you want by casting and re-loading the save.