Thanks! I've seen some Skinwalker PrCs, but most of them were either based off of the Dresden Files' Naagloshii or didn't have as much flavor as I wanted.
I didn't want to give higher than 5/10 casting because the closest analogue is MoMF, which doesn't have any casting. The MoMF isn't as limited in shapes as the Yee Naaldlooshii, but the YN isn't limited by HD, only by what it's killed. That means the average YN will have a skin or skins maybe 7 CR above her level.