[FFnut]

Impossible. The main problem between the two joining is that their plans conflict - Ultimecia's plan requires she be the only person left alive, and Kefka's plan requires that everybody else die horribly painful deaths. As for those "tier" things, they're simply his last defense before he fights you himself. You're close enough to him anyway, so he figure he'll give you one last "ladder" to climb before he crushes you. Hence the tiers. He's basically just softening you up (as if the goddesses didn't already do that) before he tries to finish you.