Those theories can't really be correct, because didn't that Island thing happen before Sora lost his heart?(when Roxas came to be) That would make Xion a higher number than Roxas (unless that is somehow true and will be explained in the game, idk).
Not necesarilly. The numbers indicate when they joined the Organization, not when they were created.
Xion could've been created then, but either
1) Wasn't found by the Organization so she couldn't get drafted and given a number
2) Refused to join until after Roxas did (it doesn't seem to be mandatory, though possibly leading to capture)
3) used as a guinea pig like Namine was in CoM until they had a reason to let her join