From what Tetsuya Nomura has said, I get the sense that KH is sort of in a different reality from FF. I mean, just consider that Tidus and Wakka live together as children along with Selphie, while Squall seems older than he was in FFVIII. This doesn't make any sense whatsoever, even if you try to use the Sliders theory wherein there are an infinite number of Jerry O'Connels trying to find their way home in an infinite number of universes. Even then, there's too many inconsistencies for it to be the same FF characters we know and love. This way, if Cloud DOES hook up with Aerith, it will be KH Cloud hooking up with KH Aerith, not FF Cloud hooking up with FF Aeris. Now, FF Cloud hooking up with KH Aerith..... who knows man, who knows.....