yeah, smaller gars are usually pretty peaceful. as long as ur black ghost knife isn't like 1.5" or something. jus try them at about the same size. should be okay.
make sure u get the Knifefish a clear tube to house in. DOnt waste your money on a crappy plastic tube, get an old candle chimneer (glass) and drop it in. looks 100 times better then the crappy tube or pvc pipes.