In regards to the second question:
It's actually not that hard to make new models. Anyone with enough 3D modeling and coding experience can do it. I've done it myself with several feline ears and tails.
Like Jango_Fett said, the biggest problem is that you can't simply add things like new body part models into the game (as far as I know). You have to overwrite a preexisting model to add in a new one. If you want to, say, make a new tail, you need to choose one of the tails already in the game to replace. You also can't rename them in any way. If you make a feather tail to replace Ftail5, it's always going to be called "tailless" no matter what you do.
Still, given that it's possible to replace ears and tails, it should (in theory) be possible to replace the entire canine and feline models, but then you run into another problem: the difficulty of actually accomplishing such an enormous task. It's much harder to modify, convert, export, and properly import an entire model than it is to do the same with just ears. The process of importing and overwriting models involves finding very specific blocks of code in your original file and replacing them with new blocks from your exported file. The more complex a model is, the harder it is to accomplish this without messing up. If you don't know exactly what you're doing, it simply won't work. Heck, sometimes it doesn't work even if you
do know exactly what you're doing.
^ this is an example of what happens when the code isn't 1000% perfect ^
ALSO, ofc, what we can replace is limited by what we can access. While it's possible to replace the bodies, ears, tails, and manes, I don't believe it's possible for the average person to replace the wings. There isn't a publicly available .blend file that includes the wings and their skeletons. Without those, the best you'd be able to get is an item.