The theoretical tie between these games is that each is a story in a set timeline: past, present, and future. It doesn't follow from past to future, but each is like an episode that occurs in different times. Therefore, only a couple of magic system changes need to be explained.
Anyway, what's right is right, and FF games aren't supposed to be connected. But if they were, how would it work? That's basically a topic reminder.
For anyone who's ever wanted to see all their favorite FF characters in a single game, just recall, that's what KH is for.