Jecht is technically Sin... So I feel it still works since Tidus' spent most of X trying to defeat Sin.

These two have more of a connection really, that's why I feel he was chosen. Besides, Seymour is a lame villain and the Villains had enough mage characters.

Technically, Cecil should be fighting Zemus/Zeromus but Golbez was chosen instead because of their antagonistic relationship.