^Not necessarily. He never set in stone a limit, he just gave out what will most likely be the number of third parties.
That said, Tails would be in before Knuckles. Why? Sonic and Tails was the original duo, then Knuckles came later. Not to mention that Knuckles usually does his own thing to help the team, while Snoic and Tails lots of times stick together, although they do split sometimes. =P Knuckles only chance would probably be if they had updates to the game.... Which would be cool. We don't know if they will, though, and even then the chance is only decent at best... Although if sonic is in, it would be HILARIOUS IMO if Tails worked sometimes with Sonic for recovery and a special attack or two... XD Too bad these boards would be FLOODED with cheers and complaints then. ;_;
Although personally, I thought of G-Red when I saw the gliding update. Meh, I have a weird brain, but the gliding works really close to his Booster jump. He probably won't be in, though, because of the low support, but I'm just saying how meaningless it is to see an update, and go 'Knuckles has a higher chance now!'
Sadly, he doesn't. It was mostly decided during the design process who would be in, IIRC. I think gliding was made to accomodate those who COULD glide, since it just makes sense, IMO. =P