Yes, quite an interesting theory. And I like to see FF characters related somehow too.

One thing that could make or break this theory, is the fact that Laguna would have to be around Cloud's and Tifa's age.....

Which means that if they were actually from the same planet, then Laguna, being a Galbadian soldier, probably would have been a soldier in the FFVII world. Perhaps a member of the Shinra's Soldier?

But then again, it is two completely different worlds. However some similarities are present..........