Uh, well. I beat 20 guys (total) in cruel melee just to test this, and it didn't work. I can get up to 6 as Kirby, but that's about it. It might be cool if Sonic and Tails were in SSBM, since Sonic Adventure 2: Battle is really fun, or something, but it's better that they aren't. They wouldn't fit in. They'd have none of their own items, no stages, nothin'. So, uh, yeah.
Born To Play:
"What if everything you see is more than what you see– the person next to you is a warrior and the space that appears empty is a secret door to another world? What if something appears that shouldn''t? You either dismiss it, or you accept that there is much more to the world than you think. Perhaps it really is a doorway, and if you choose to go inside, you''''ll find many unexpected things."
-Shigeru Miyamoto