The Iifa tree is not really involved with Kuja's plans, it deals with soul manipulation but that has more to do with Garland and Terra than Kuja.

You are right about Kuja knowing about Trance before the scene with Mog though. He does mention Trance with some familiarity though I can't seem to find out where he learned about it from the script. In the scene in question, Kuja finally figures out how Trance works.

Kuja: "So...an eruption of anger against one's surroundings induces a
complete Trance! It's not the will to live, nor is it the desire
to protect another!"

Kuja: "That means...I simply need a powerful soul, even if that soul
is not my own! No, wait... And it can be found there! Hahahaha!!"
The script and other sources pretty much states he finds these strong souls in the Invincible.