i used to have 3 big black ghost knife - averaging at 16inches. Sad to say I sold them. They are all living on my community tank with rays and bichir. Currently, I have 10inch BGK that I was expecting to be eaten but still alive and swimming alongside with my rays.
I think, if they are big enough (though i have some 3-4inches catfish) and have hiding places they can survive. Mine likes to hide in between with the branches of my Frankenstein Bonsai. It only swims out at night and feeding time.