That's where your research goes wrong. If you want to test who's the stronger, you're going to research the HP, and stats of every monster and enemy in all of the FF worlds and then compare them to each world individually, and then compare each world individually to the Kingdom Hearts 'multiverse'.
...no I don't. Stats are a gameplay element and reveal nothing as to how strong they are plotwise. Ruby and Emerald Weapon are not stronger than Sephiroth, despite them having stronger stats.

Also, what I was pointing out was that Kingdom Hearts is set mostly in a Disney setting, it has little background for itself, it doesn't even explain how anything occurred (though im sure it will if it hasn't, I haven't played BBS nor do I plan to).
I'm not seeing how this matters.

Even if Sora killed the most powerful being of that 'multiverse' it has no adverse effect on the other worlds, because the villain of that 'multiverse' doesn't exist in the world of Final Fantasy
Again, not seeing how this matters. Plus, Xemnas is stronger than the other FF villains because he is "the strongest being" on a much larger scale.

Please, I have to ask which you're talking about. Are you talking about Xemnas? Because if you are, he did not beat Xemnas single-handedly.
YouTube - Kingdom Hearts II - Xemnas 1 (Boss)

The stats of Kh are linked to ff, and if you notice, sora never nears the stats of ff heros, and neither does xemnas, his stats are pretty low....
Incorrect.