I believe their reasoning for having Jecht for X and Golbez for IV is because they are much more physical compared to the more important villains from their games. Seymour was purely a magician/summoner, so he wouldn't be able to put up his dukes in the manner of Sephiroth or Kuja. Zemus was also frail in human form, and his other forms are probably not humanoid enough for the game's fighting engine.

But if my theory is true... Choosing the Dark Cloud instead of Xande makes no sense. She was a giant torso coming out of the void.