Um... I wouldn't put a knife in with your other fish. The black knife, while more passive than its relatives, is still a "hunter" by nature. Your betta's pretty tail would be way too attractive for it; and the guppies would just be food. It would do better with either a tank of its own, or with fish that are larger than it until its grown. By that time, it will not consider the other fish as prey. I may be new to the betta world but i've had knife fish before and speak from the loss of my angels... knife fish do not play well with others.