There's no reason not to put them together. I kept my bgk until about 13" with s/c americans, and even a big bastard D. compressiceps with zero problems. As said, give the knife it's own retreat. Not that the oscars will attack them, but that's just how ghosts are, they like their hiding spots. Also just make sure the knife won't fit directly into the oscars mouth, but that's kind of common sense.