Well we have YU YEVON!!!! Since person "X" and FF X both have a "X" in their name. So Yu Yevon posses Ulti = Edea and started all this,ya?
And the heros start a pilgrimage to defeat Yu Yevon which is Ulti, right? So FF VIII and FFX is kinda related, ya?
So the girls who are sorceress = summoners
The guys who are knights = guardians
As for the conditions, simple! Yu Yevon can only posses aeons in FF X but in FF VIII, only sorceress got possessed. Not only that, Yu Yevon got defeated by SUMMONER in FF X, so he hold a grudge and developed a way to possess summoners which is sorceress. There is also a high possibily that Yu Yevon is actually HYNE!!! I am working on the theory now...
I am smart, ya?