Sonic and Eggman in SMK!

[NICKtendo DS]

http://www.spriters-resource.com/resources/submit/

Um...Doesn't this break the rule that if it doesn't come from a certain game, it's not allowed. IE Sonic wasn't in Mario Kart and shouldn't be accepted by that fact.

Or am I mistaken?

You are partially right. If it doesn't come from a game, it's not allowed (custom characters, etc.).
However, Sonic DOES come from a game. It doesn't matter how you make it, for what game you make it, as long as it's usefull enough, it is allowed.