Eh, they aren't any better integrated into the first game. Although I do recall "Leon" saying something about not using the name Squall any more because Squall lost someone important to him, the implication being that Rinoa was either lost or separated from the others when the Heartless got their world. (Also, in the first game, Leon's VA was David Boreanz, which was just too perfect. The substitute in KHII was not nearly as right.)

I once started a fic that brought a good chunk of the FFX/X-2 cast into the world of KH, but it never quite found enough of a plot to keep it rolling. I still think about going back to it though.