We currently have four 5" blood parrots, and a 3" blood parrot, also a 5" plec. The parrots dont bother the plec, and I was kinda hoping a knife fish would be ok in there too.
I think regardless of tank size, the knife would get picked on. They're really shy and slow and pretty well defenseless, and parrots have bad reputations with compatibility anyways. I think even a larger black ghost would get attacked in with parrots, especially since they're already so big.