I have keep mollies, guppies, and mosquito fish with FBT's before. Tough little fish like that will be ok with FBT's as long as you consider it acceptable for the occasional fish to be caught and become a snack for the FBT's. The FBT's will try to catch the guppies if they swim nearby but they won't pull it off very often. Don't keep any fish with them that isn't both edible and expendable though, they will try to eat nearly everything that moves including human fingers and each other.