Hello,
I'm afraid ABSOLUTLY nothing can pretty much be with fbts. As said there aggresive, and have a special toxins. Which will most likely hurt any animal kept with them.
The only other live thing I can see being put in the tank, are aquatic plants, and maybe some feeder guppies. But being that you have alot of fbt's that may not even work.
I have 3guppies in my fbt's tank. Thats from trial and error, birth(AND LOTS OF IT!!!), food,giving some away,death,etc...lol. The ones left are a mother, and her two little babies, that if were any smaller she'd eat! I started off with a dozen.
I also, once tried some goldfish, well to say the least those didn't last very long,lol.... hmmm maybe a day or two.
Steven