I don't see any of the games sharing the same world personally. X and VII's world seem nothing alike and i don't understand how they could be for one Spira is a lot smaller than VII's world. Planets get smaller over time not bigger. 2nd those two games have different involvements for death. Lifestream for VII, Farplane for X. There are no pyreflies in VII nor is there a lifestream in X. 3rd there are different races in both games. There are no Al Bhed in VII, no Guado, Hypellos, duck people, or Ronso. There is no whatever Red XIII is in X either.

In our world names of countries change over time becuase of war and other reasons, but we still have a few countries whose names haven't changed for hundreds of years. VII doesn't have not one location named after X's world.

I can do similar comparisons for VII and VIII too since there is a similar issue.

The only ones that I can possibly see being the same would be any of the Gaia ones, but if they were all connected I couldn't see how the story could flow right in Dissidia. It's basically about a demon who has summoned a group of the best evil fighters from around the universe to do something evil and a goddess counters him by gathering a group of good fighters from around the universe to stop them from whatever they're trying to do.