Hey just a fast hint:
If you make Muties and Ghouls have diferent stats people will en up making only especialized powerbuilds, and useless Classes (lets say Ghoul is useless to fight: nobody uses Ghoul skin).
So make it the way of Traits: Ghouls, muties everybody has diferent traits, including some human ones, humans have special traits too.
If you want to play a slow but very resistant (and idiot) mutie: choose a trait
if you want to play looking as a beautiful mutie, but without the stupidness just dont choose a trait and use a normal powerbuild.