I can imagine FFT coming first, but not inbetween them. Personally, I consider FFT and FFTA/FF12 to be different universes altogether.
What connects FF12 to FFTA is, mainly, Monteblanc. He exists in both games, after all, and looks pretty much the same. Either moogles live a long time or he's just an ancestor to the FFTA one is debatable.
My guess is that the world of FF12 was laid down in a book and, over time and space, began to pick up a sort of 'real dream' aspect to it. Like an artifact in D&D.
My brother got FF12 on Halloween, and I've been watching him play it on and off. Just from observation my mind has been changed by it, and I find it to be a fantastic game. The turning away from the previous ATB system was just NEEDED, and the Gambit system doesn't just sit there and play it for you like some may think.
...but am I the only one who thinks, with a bit of tweaking, it could have benefited from a two-player mode? I kinda like having my RPG's two-player, being one reason why a friend and I played the shit outta Legend of Mana, and I can fully imagine FF12 doing the same.