I always thought the FF characters felt out of place and was there more as fanservice than anything else. I liked the few FF characters that appeared in the first game, but even then they didn't seem to have as strong a stage presence as the Disney characters did. I would have liked to see actual Final Fantasy worlds be thrown in with the Disney ones to make them feel more at home, but that never did happen to my knowledge. I only have the first game to go off of, but if you removed most of them you probably wouldn't miss much.