I have a black ghost knife with some very large severums, blood parrots, jack dempseys and oscars and not only does he do well, but he thrives. he's right up in there grabbing food with all the cichlids, squeezing between them, and grabbing all he can. The cichlids don't really acknowledge him at all.
Now, that is my tank. Just because I have success, doesn't mean you will. part of the reason why I may have had success is because all these fish grew up together. Putting anything besides cichlids in with cichlids is always a risk.