I gotta say, this is a fantastic idea. I've been working on some Final Fantasy stuff of my own, and this FFX d20 project (and your Limit Breaks) are definitely going to see use in my campaigns.

One thing troubles me, about the Aeons: if I remember correctly, Monster Classes have to be played from start to finish (it's been a while since I read Savage Species, so I might be wrong) before you can multiclass. Just looking over the Aeons, would I be correct in assuming that they can multiclass freely between the Monster Class and the Prestige Classes?