This is funny because I'm the opposite. I LOVE large casts of characters, and in X I used every character in every fight

I enjoy using multiple characters so I just put in extra time grinding them all and I always take the lowest level characters with me when I progress the story (meaning during times I can't just switch them out whenever). The enemies and bosses in those areas usually give good experience to catch them up.

VIII is more about junctioning like WK said, so I wouldn't worry about having to catch any up. There's a little bit in IX, but they make it pretty easy to get them caught up.