Of course 6th generation battling is going to be exclusively between new games. The same way you couldn't battle between 4th and 5th, or 3rd and 4th...
Trading, my bet, is going to be rather a "migration", so once you transfer your pokemon from 5th (and maybe 4th) generation, you can't go back. Just think like a reversed Pokemon RAdar.
And I bet they're going to keep the surf/fly/cut and whatever HM they come up with, the same way as always, albeit, with a fancier sprite. It would be insane to try to customize every single pokemon capable of every single HM, and then some. But who knows...