I've got a question: What program will you use for this game? There's a great fighting game engine, calld M.U.G.E.N. If you interested in it, I'll give you the a link...
The sprites are not bad... But some of them needs to fix. The outlines of jazz's fur of his stomach must be darker, and the upper part of his head shorter. Sonic's nose part (the pink nose part) must be shorter too, I think.
Good luck for this game!