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.
Mu'gel'es va'ne kanok-vei glah-tor n'ash-veh estuhl. Nirsh olaya kusut seh la-tushta el'ru t'nash-veh mura tricheq. Dung-gau shetau yeht da'kum