Umm...i'm not to sure what you were trying to say there, but it's probably just the syntax. I think i might know, so this MIGHT answer you're question.
I'll be using a party changing screen available in certain scenes and moogles. THere will be a temproray "native" character in some of the later dungeons, similar to Kingdom hearts. Actually, there more of Optional Gaias than dungeons. ATM, it'll be Irvine Kinneas in Gaia VIII, Cloud Strife In Gaia VII(largely because i have all the graphics ready) and Gaia VI's character is pending. i may use further Optional Gaias, but the only ones that I can confirm are:
Gaia I (only kingdom of Cornelia)
Gaia IV (A fairly major one, with a few side missions)
Gaia V (brief, if at all..)
Gaia IX (Major world, althouogh will have a few changes from the original)
Gaia Ultima (End of Game phase. Can still revisit some Gaias.)
Gaia VI, VII, VIII( optional worlds, with high level monsters. Set in an altered version of their plots, simialr to the disney worlds in KH. Petros is desiging Gaia VII at least, save a few cutscenes)
Edit: I will be using an FFIX style skill system.