Tails would have more than average speed (expesially in the air, more so than Jiggly). Dont forget, that one of the reasons Sonic chose Tails to be his companion or sidekick was because of his ability to keep up with him. This is stated numourous times.
Therefore, his speed in the air would Almost match Sonic's running speed. Ground-wise, not so fast, but about average. He would be weak on the ground, but would have fantastic aerial moves.
I could see something like that. Maybe an extension of peach's floating jump, only with more horizontal speed? As for those aerial moves, his tails could give him some great range, and he could have the same 'Attack after upB' that Sonic has.
I would imagine Knuckles to be very similar to DK in terms of moveset, he would be perhaps much lighter than DK, but stronger, and slightly more speed. Gliding would be available, but I dont know how his climbing would work, perhaps it would be an improved version of wall jumping?
Mmmm. . .Stretching it. Lighter and faster than DK, yes. Stronger, I dunno about that. Dk deals a hell of a lot of damage, and I don't think Knuckles needs a speed and a strength advantage. Additionally, he should have a rather slow attacking speed. This is hinted at in Sonic Battle. So basically, somewhat like Ike, but less so.
As for the climbing, I don't see why anyone thinks this is a big sticking point. Peach is the only one with that floating move, Kirby's the only one that copies, and Sonic's got the after-recovery attack. There are lots of characters that have skills that no one else has. Why would knux be different?
Shadow would be very similar to Sonic, but with perhaps more powerful attacks, but slower. That said, his running speed would be about the same. He could easily have a differant moveset to Sonic, but the two characters above are more deserving.
Running speed would definitely be less. Look at Sonic 06. But then again, slower-than-Sonic is a pretty broad range, so that's not really an issue. I don't see any reason for his attacks to be slower than Sonic's, though. Just Luigified. And even though he could have a different moveset, she wouldn't. He was a Sonic clone in the first game he showed up in (Practically, not storylinewise.).
All three would most likely be ATs, but you never know...
I dunno, Sonic is a huge franchise. And it wouldn't take that much work to code in Shadow. Clone characters are easy to make, after all. But I'd rather see Tails before Shadow. Knuckles, though, is AT fodder at best. Unfortunately.