I wouldn't be concerned about them eating the knifefish but I would be concerned about aggression.
Both the pikes and the knife are territorial and both like caves and places to hide. I would be more concerned about the knife attacking and harrasing the juvenile pikes at this point. Eventually both fish will top out at around 14" and then the pikes might have the upper hand.
I have kept Saxatilis with BGK in the past but it was in a 200 gallon tank heavily decorated with lots of lace rock. Even then the larger pikes would chase after the BGK whenever they ran accross each other.