I have a Black Ghost Knife with a few Angel fish and a Pleco. When I bought the Ghost it was about 3" long, the Angels were about the size of a U.S. quarter, and the Pleco was 3". Now the Ghost is about 8" long, the Angels are 6" tall (fin to fin, bodies are around 4" diameter), and the Pleco is about 5". Never had any problems and it has been a couple of years now.
The Ghost will eat guppies up to 1.5" long easily. I usually toss in a couple of dozen feeder guppies once a month as a treat.