Cyan is one of the stronger characters early in the game. His dispatch ability is one of the deal more damage than a normal attack. Pummel and auto crossbow are the other two.

There's very little added to the story by choosing particular characters because it was designed moreso that each one has separate side content to explore. Like there are a couple more lines depending on the team you take to Vector but nothing significant.

The entire second half of the game is comprised of their individual stories. Technically there are only three characters you must have in order to beat the game (Celes, Edgar, Setzer) which happens to be the minimal needed to form three teams to complete Kefka's tower. Everybody else is optional.

Gogo, Mog and Umaro are the least involved characters in the story. They're all ancillary and you could play the entire game never meeting them. Although if you think Gogo is pointless you clearly haven't checked his/her status screen.

Gau is the worst character in the game since his only function is a berserked mode of random skills that mostly suck. Everyone else has a function. Gogo and Umaro are the best competitors for the coliseum.

My FF6 tip for anyone interested: don't equip espers on non magical characters. It keeps the cast far more diversified if everyone doesn't have Cure3, Haste and Ultima. It gives more incentive to use less popular characters like Strago and Relm, and has the bonus benefit of better balancing the game. FF6 is probably the easiest game to break and end up doing 9999 damage with every type of attack thanks to all the stat gains from espers. Subsequent plays were much more enjoyable by keeping the characters from becoming so overpowered.