1) There's a mermaid shape here by Marvine/Beosboxboy that I strongly suspect is showerproof. It could be retextured as snakeskin. You can also try the one on MTS, if all the meshes are included, you shouldn't need any particular EP.
2) I've seen satyrs on various sites.
3) You just need BodyShop (comes with the game) to retexture a skintone. There are several tutorials for the basic process, and several more advanced tutorials for special needs (such as BBB's Pixel Perfect Skintones). Try:
http://www.thesimsresource.com/tutorials/view/category//id/2575/bodyshop-tips-and-tricks