I don't know about the snakehead... Probably start chowing down on the other inhabitants and I've read that pike cichlids have pretty bad attitudes as well and once they start breeding I'd feel sorry for the knife fish.
k i think ill stay away from the snakehead and pikes. But just so u know the knife gets almost 2 ft long (around 18"). Think i mite get a pair of true green terrors and raise them all up together. That way it mite work.