...uh, you want Maxi pad from Soul Calibur? Elvis? That Maxi?
Wow.
Anyway, as I say all the time, of COURSE Shadow can have a completely different moveset from Sonic. But he absolutely will not be appearing in this game. As HiddenTiger already explained, there simply isn't room for him. If they're only including 2-3 third party characters total, it means they're going to be choosing special characters. Characters that have impacted video gaming as a whole and that most if not all gamers will immediately recognize and characters that they know and love.
Shadow does not have that kind of clout. He is entirely too new and he should have stayed dead in SA2 anyway. Solid Snake is a character like the one I'm describing. Others include Pac-man, Sonic, and Megaman. Not Shadow.
Normally, people tell you to "never say never," well, I'm saying never. Shadow will NEVER appear in Brawl. It is absolutely not going to happen, I promise you.
He won't appear as a unique character anyway. Sonic could have a costume to look similar to Shadow, but that's about it.