I've never played Sonic Shuffle, but if it's a Mario Party ripoff like I've been told, it should work fine if they did it right, and it doesn't matter what characters they use as the onscreen avatars. The players in Mario Party are really just there to give a thematic design to the visuals, and the gameplay in Mario party is just using those Mario themes to effect in minigames.
However, in RPGs, the characters are absolutely essential. Mario Party would work fine if you ripped out all the characters and replaced them with, say, Sonic characters. The gameplay has no real plot and is just a bunch of minigames. However, in RPGs, the characters are essential to the gameplay, with character development and plotline being a major part of the game. If you ripped the characters out of Final Fantasy and placed Sonic in there, it would feel totally out of place, and honestly, rather stupid. I'm sorry, but no matter how much they try, Sonic plots have and always will be kinda cheesy, and for an action game, there really isn't much of a problem with that. Cheesy plots work fine for an action game. However, for an RPG, it just doesn't work. Imagine Sonic and company actually being serious about Eggman's latest scheme, instead of joking around and just beating Eggman again.
There are just an incredible number of reasons why "Sonic RPG" is an oxymoron. It's a bad idea. Even IF SEGA was a decent developer capable of making new Sonic games that weren't shovelware, I don't think a Sonic RPG that doesn't suck is even POSSIBLE, never mind doable by the glut that is Sonic Team nowadays.