How did he get from Balamb to Hollow Bastion? Why would he go there? If he has his scar in KH and came from Balamb, then that means the events of VIII happened, which is impossible because of the Selphie factor. Aerith talks about living together, all of them, for quite some time.

Why cant he just be a character set in a different universe from the one we're used to, like Nomura said? Because...? Why?

Edit: Further proof exists in Tidus and Wakka. Tidus is (SPOILER)a dream in FFX. But nothing in the events of KH warrants him being there, much less warrants him staying. There are no fayths (at least that we know of) and no reason for Tidus to be there if he existed in the same universe as X.

Has anyone ever read Tsubasa? It's a Clamp manga that takes the main characters of a few different mangas (Card Captor Sakura and many others) and puts them in a whole different universe. It's a story with characters we know, but it exists apart from their normal stories. It's a very good example of the differences between KH and FF.