This is a really good idea, I might make their laws very simplistic. I might even steal the ones from Animal Farm.
In my mind the mongrelfolk definitely won't be inherently evil. Paranoid or scared of outsiders, definitely. But they should have near-human intelligence, but slightly diminished.
Professor Moreau to me is the evil one but purely because he doesn't hold much regard for the lives of the people he has experimented on, especially in a world with polymorph magic and Druidic wildshape. Though Moreau will be helpful to the players and give them no good reason to kill him, since they're his chance to regain control of his lab.
I've been going back and forth on if Moreau will want the mongrelfolk dead, since they'd hold more scientific value to him kept alive.
On the off chance that happens I'd probably give them one of the beast traits, probably one of the ones that's less useful like keen smell or web sense