Putting it all together, I feel there is sufficient evidence that there are at least three different worlds that the FF series takes place on:

1) Spira (FFX, FFX-2)

2) The Planet (FFVII)

3) Ivalice (FFXII, FFT)

Also the events in Spira would logically predate those in the Planet (although if we involve time compression, cause and effect may be thrown out the window). I feel strongly that the events of FFXII predate not only FFT, but FFVII as well (since it seems that FFVII and FFT happen at the same time).

As for the other games I see no way (as of yet) to connect them to each other beyond shared themes.