Dragonbreath
Smash Ace
First off, Slippy is the techie. Not Peppy. Peppy is more of a veteran/father figure, and I can't really think of a parallel for him in Sonic.Hm... I just thought of something.
Sonic is to Fox (Hero) as Knuckles is to Falco (Independent) as Peppy is to Tails (Tech help) as Slippy is to Amy (Annoying) as Shadow is to Wolf (Rival).
Which Star Fox character did we get? Falco. So Knuckles is probably more likely.
Anyway, you're forgetting that Sega would monitor the inclusion of their property in Brawl very closely and have a big say in who gets what, representation-wise.
Also, there's not as big of a difference in popularity between Tails and Knuckles as there are between Falco and Slippy.
If I wanted to apply your idea to another comparison, like Sonic/Mario, I could say that Tails is getting in because Luigi is. If I wanted to, I could even say that Amy, Eggman, Big and Knuckles would be in because they compare to Peach, Bowser, D.K. and Wario. But that wouldn't make it true.
True. If anything, Tails is more similar to Fox. Flies a plane, uses weapons, is. . .well, a Fox. . .I just realized that Star Fox is like Sonic because Sonic came first, but they really aren't alike.