Sephiroth's Supernova and Kuja's Ultima are completely different scenarios. Supernova was a summon invented exclusively for the Safer Sephiroth battle, Ultima was used in not only a cut-sceen, but an FMV as well. To me this makes Ultima very much a story attack.

And yeah, Zidane would totally be more powerful than Vivi or Stiener "irl", as such would be.

As for Sephy versus all 5 weapons, it would kind of depend on location. I don't see how 5 weapons could effectively fight against something as small as Sehpiroth. If we are talking northern crater it would be more like a series of one on one battles, and seing as I am quite sure Sephiroth is smarter than the Weapons he would probably come out on top. Versus all 5 in the open? Well, they were created eliminate Jenova, so I am going to guess the planet thought 5 was the magic number that was strong enough.

Kefka vs. Everyone. In terms of power with the Light if Judgement I would say Kefka has them beat, but if it is just the 11 of them lacked in a room I don't see him comming out on top. I mean, that would make each of them weaker than your individual party memebers (who teamed up and beat him), and I don't think your characters are supposed to be that strong by the end of the game.

Though Kefka acquires a great deal of power as the game progresses, he never seems to be a great fighter. He is more an accumulate power and only strike when you have a plan to win type.