Not crazy at all really. Knuckles and Shadow are of course already accounted for as ATs so it won't happen this time around but realistically that'd be perfectly doable. We already have precedent of similar characters and groups of similar characters like shotos in SF aren't anything new. And in general characters with a common theme have always been a thing in fighting games. In Smash we've already got:
Mario Bros.
![Mario :ultmario: :ultmario:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Luigi :ultluigi: :ultluigi:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Dr. Mario :ultdoc: :ultdoc:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Space Animals
![Fox :ultfox: :ultfox:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Falco :ultfalco: :ultfalco:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Wolf :ultwolf: :ultwolf:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Lords
![Marth :ultmarth: :ultmarth:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Lucina :ultlucina: :ultlucina:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Roy :ultroy: :ultroy:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![Chrom :ultchrom: :ultchrom:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
(this one is ended up working out pretty well funnily enough as the remaining unique FE reps aren't lords)
Though third parties are allowed echoes perhaps a full set of 4 Sonic characters may be considered too much, but given how popular he is, especially among Nintendo fans, and the close relationship Nintendo has with SEGA, it could work without using up too much time or resources if they were so inclined to give it a shot.
They all have their own unique traits and abilities but when you break it down, Tails, Knuckles, and Shadow can already do most everything Sonic can in the actual games so basing them all on him would work well.
Tails and Knuckles would probably need a few more changes to their normal attacks, but they'd definitely work as semi-clones with Shadow working well as Echo even if he got a few changes as well. Though the line is a bit blurry when
![Ken :ultken: :ultken:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
is an Echo but
![Dr. Mario :ultdoc: :ultdoc:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
isn't.