^ Well, i haven't tried a puffer with any of the Knife's but i do have two palembang puffers living with a (roughly) 20inch fire eel without any noticeable problems in a 200G decorated tank with fine sand base, plants and driftwood...
Although i have to be honest and state that when mixing any species of puffer with other fish you are gambling the life of the other fish, or both (if puffer gets eaten)