They decided to not put in any of the FF characters from the other FF mainly to focus on the stories of Aqua, Ven, and Terra, and also why they had Zack in this game is because he was the main character in A prequel, and because this game is a prequel, they put him in it.
Not sure how you couldn't get an answer It's easy to find out just by looking at the Wikipedia page for Birth By Sleep. Yes, as they said above, only Zack Fair from FFVII and Crisis Core is in Birth By Sleep. As he was the only one who would have been old enough at the time of the game to actually be any help. The rest would've been around 10 years old or so.
Wow, I've beaten Birth by Sleep twice and never took notice to the fact that Zacks the only FF character in it xD That's pretty weird, but yeah I agree with everyone else KH Wiki is pretty reliable imo and would have been a easier and quicker answer.
I think because KH BBS is a prequel they didn't incorperate many FF charactors, I guess from their perspective charactors such as Yufi, Paine, Yuna, Rikku, Cloud, and Squall were more good friends with Sora, Donald, and Goofy. Im not really sure the guys knew Ventus, Aqua, and Terra that well. Hey did anyone notice how small they made Rikku Paine and Yuna( they look like midgets in KH2), Also FYI keep in mind how long ago the plot for this game is Kairi's only like five. I realize there are plenty of holes in the KH series but you have to consider that some organizations were not formed yet by BBS.
It was said beacuase BBS is a ten year prequel to KH, they did not want to much FF characters. They needed someone included in the past of a FF game, and I guess they chose Zack since he is part of the FFVII backstory. So aside from Zack and a moogle or two, no FF characters. Also because they might confuse some fans into thinking there is some FF side story in the game where there is none present.