Hello,when a Beastmorph uses his mutagen with Beast Shape ability,it isn't considered polymorphing. It can pick one or more abilities from a list, depending on level. So,can he then combine that with the actual extract Beast Shape or Monsterous Physique to polymorph into a beast or humanoid with the combined abilities? For example, he could be a large animal with pounce, fly,+4 STR,-2 DEX,+4 Nat armor (stacks with +2 alchemical Nat armor).
Aside from the natural armor not stacking(the mutagen bonus is a natural armor bonus, not alchemical), I don't see why it wouldn't work otherwise.