He does Rolling Smash, which granted is a flip, but it's in no way graceful.
The best comparison I can think of is that Rex uses the Aegis Sword as a strength weapon and Pyra uses it as a finesse weapon. There are some similarities in how they fight since they both use the Blade Specials and such, but they would definitely play very differently in Super Smash Bros, especially since Rex & Pyra/Mythra would likely be much slower overall due to a higher focus on Xenoblade Chronicles 2's battle mechanics.
I doubt it. There's no one he could echo, and outright replacing Pyra and Mythra with Rex & Pyra/Mythra is about the cruelest thing you could do.
If they do anything, they'd add him alongside the two, but that would 1, be another addition of the same character on the roster so very few would get behind that, and 2, never happen because they need to represent the newest game in the series.