Actually, I wouldn't be surprised if the FF characters were in BBS in general. I mean think about it, in KHI, aerith and leona and yuffie talk about how they knew Ansem in their old world, and Ansem is directly related to Xehanort. So considering MX IS Xehanort (cause also remember how Ansem said he found Xehanort without any memories), I mean, it wouldn't really be a surprised if the FF characters showed up here and there, even if they were just kids. And also keep in mind that even though they retain recognition of related FF characters, the FF characters thus far in the KH games have relatively completely different stories. I mean, just the fact that Aerith is ALIVE is one example, and then all of the characters knowing EACHOTHER, even though from different games.
So FF characters showing up in BBS is completely believable, but they'll be like the ages from 10-12.
One last comment: GODDAMNIT i WANT A PS3 EVEN IF IT IS ONLY TO BY THIS GAME!