A more flexible animation system

There is also extra bones (the wings) on the smaller character that the taller character doesn’t have.

Well, you’re buying models that aren’t made to go together. I would assume just renaming the bones & matching the rest pose would work as long as the general structure (i.e. hierarchy) is the same (e.g. spine_1 → spine_2 → spine_3 → clavicle_l → upperarm_l → etc.) Plus, extra bones (like ponytails or extra arms) are alright.