shadow... FYI, you need to tone down the HP on your bosses, and make the sprites only cover areas where you can actually hit. If you want to make a boss hard, then SOC him some new attacks! It's not that hard, you just need to know how to use the editor(Soc and text). You could easily give bowser a short firebreath attack if you tried. And as for the sonic thing, don't be so up tight about it. Sure, it's a mariokart mod, but its for a sonic game. You can sneak in a sonic character! Really, I'd just be happy if you made the eggman kart.
In any case, I've actually seen a mariokart sonic kart(it was a hoax), and it looked awesome.