Knifefish are a little on the shy side, as long as they re roughly the same size it should be ok. Peacocks like a higher temp than knifefish though, and knifefish like water a little on the soft side.
Just make sure the pbass isnt out competing the knifefish for food as they are most active at night, so you may want to consider two feeding scheduals. I currently have apbass in my growout tank that will be joining my rck shortly.