Honestly, I dunno much about how models with different parts are made. However I do know that models can have Body Groups to make use of multiple parts of a model and Override materials to recolour specified textures on a mode.

That said, what if you can do the same with the Avatar? A Custom Avatar pack could come with a model for each animal and gender. The accessories and clothes could be seperate props but you on SFGM you can align and lock onto the character and use Overwrite Material to pretty much recolour the models and props. (So you don't need 3 of the same shoes in different colours when it can be the same models but with colour scales.)

That way, if someone wants to use a specified Sonic Forces Avatar thats not provided in the Avatar Mania pack, they can make it themselves. And perhaps go as far as to decorate them with other cosmetics from TF2 or other SFM downloads.

Just a thought.