I used to have a black ghost knife (named Finley). His tank companions were adult guppies, a clown pleco and six common hatchets. I never had problems with guppy over-population until I rehomed Finley, then I was overrun with guppy fry. My friend who took Finley had a nice school of neons until she put him in the tank. Within a couple of weeks, Finley had eaten all her neons. So, based on our experience, I would suggest that you not house a black ghost knife in a tank with small tetras or small fish of any kind.