I had a red tail shark that I bought at the same time I bought a black ghost knife. Both were tiny, the red tail shark looked like a guppy and the black ghost knife was just 2 inches at best. Anyways i intro'd them into my 29 gallon tank and the BGK took to this castle I had in the tank while the shark patrols the rest of the tank. There was other fish in there too, mostly school fish. What I found was that the black ghost knife was scared to come out of his castle since the shark would chase him around. I guess because they're both black, but at night the shark would chill out and thats when the ghost knife was more awake anyways so it worked out ok. Truth be told, I thought the red tail shark was more of a stress to my other fish than the black ghost knife. These two lived together in that tank for 4 years, then I lost the red tail shark =(
a few days later the ghost knife realised he could leave whenever he wanted, and thats when i noticed small fish were disappearing! so after all that time of thinking the red tail shark was the bad guy it turns out he was keeping all my other fish safe.
So I guess you can say the red tail shark will keep your ghost knife in check before he decides to make meals out of your other fish.