Well I'll first start off by saying that I must remove my argument about this going against the creators wishes as I have yet to find a quote. I was wrong Bolivar.

Though I find it amusing that several fans have heard of him supposedly saying it yet there is no documented proof. I did learn a few things about the man...

Favorite FF villain: Garland
Favorite FF: FFIX
Actually was working a bit on FFXII though he doesn't appear in the production credits but only in the "Thank You" section.

He likes Gears of War and Zelda.
His mother died in the middle of production for FFIII.

Back on topic...

I think it's plausible to connect some of the FF's but it's impossible (without taking a few liberties) to have a "Universal Connection Theory" mostly cause certain games are so radically different from the rest of the series mostly FFVI, VII, VIII, X, and XII.

Now while VII and X are officially connected, this still leads to issues with VI, VIII, and XII. While FFI, III, IV, V, and to some extent IX can be somewhat connected. FFII is an oddity but would most likely work in the "Crystal" FF's rather than the other ones.

The major problems are lack of the four elemental crystals in all the games, the lifestream/farplane, espers(FFVI), GF's and the Sorcerors, as well as materia and spheres.

With the different planet theory and ignoring some factors it's possible to build a timeline. In the different universe/dimension theory I feel the entire Connection Theory becomes a moot point and a same planet vast timeline theory is just improbable.