You mutate a skeleton by using Mutation or Improved mutation on it
And an Oty benefits from devouring by selecting a lower HP creature
I do not understand what you are saying, at all. How are those flaws? You can mutate any creature, and skeletons are creatures, albeit dead ones. They just get mutated into being alive again. Mutations are randomly wild things.
oty gains benefit from devouring because eating something smaller makes you stronger. Lots of games are based on that concept.