We have Mamool'ja, the Lighting Returns and other cross-over FATEs stablish that travel between dimensions is possible, so Squee can kind of pull whatever they feel like and still be "It's not a plot hole because DIMENSIONAL TRAVEL!". Introducing a whole "new" old race from another FF game wouldn't be hard at all. They can add an island off the coast (like La Noscea) and have their home there, then just say they arrived by DIMENSIONAL TRAVEL (tm) many centuries ago (or not so many centuries ago), left stranded in this weird but actually pretty familiar world. Or they can even ditch the whole "hometown" idea and just introduce them brutally like that: a portal opens, Viera (or whatever) appear, Eorzeans are super cool with that and that's that.