Just read the Novacrystallis interview

[Gilraen]

Understand that Dissidia is a Final Fantasy fighting game that uses the Void as an excuse to have all those characters available. I mean, if you look at the characters available some of them really shouldn't be there because they weren't real to begin with (looking at you Tidus and Jecht). Writers will say anything to justify being able to kick much arse with Kefka or live out fantasies of razing battlefields as Shantotto (devs love insane). Yoshida clearly isn't going to be tossing in notable Final Fantasy personalities for the hell of it, he wants to give proper justification for their appearance.

"But we've had Lightning, Shantotto and Dragon Quest golems so far, justification my arse!" But there IS justification there. In the last two XIII games you have the character jumping through space and time chasing... what were they chasing again? Meh, it was a McGuffin anyway. In A Shantotto Ascension the mad professor causes untold problems when she goes and researches dimensional magic. And the golems were just... golems made using bricks instead of rocks. The puff-puff girls fit in so well in Ul'dah you could easily forgive that they were Dragon Quest references.

So far everything's been at least permissible within the lores of the characters involved.