Someone had told me that I had to use ALL of my characters for the final battle, so I levelled them all equally. Besides, I liked all characters, so it was no problem, though, if I remember correctly, Gau and Relm weren't as strong as the others.

I didn't bother much about the spells, to be honest. I don't really like spells at all, so I just fought with weapons. :3" I think they all learned quite a few spells and I used Cure quite a lot. But that's basically all. ^^"
Also, I didn't want to have all special attacks of Gau and Strago. xD" I didn't use them much, they weren't in my main party, so they were of no use for me. I just needed them for Kefka's Tower. ^^"

If I ever play the game again, I'll probably level all characters again. It takes time but it's worth it. (Of course without all the spells and special things... I'm not determined enough for that xP)