If you plan on keeping the clown knife, I would rehome all other fish as it will most likely eat them as they grow. For tankmates I wouldn't have the more aggressive cichlids (jaguar/RT/RD) though in a larger tank aggression is less of an issue, I wouldn't personally risk it.
I'd go for large, more peaceful tankmates for the clown knife. Examples could be fire eels, larger polypterus, datnoids, tinfoil barbs, large catfish, but make sure the size difference isn't too great as you don't want any fish to end up on the menu, and remember the different growth rates of the fish. eg. CK's grow fast when smaller, so might outstrip slower growing fish that wern't on the menu when introduced, but a few months down the line could well be.
EDIT: As this is in the NW cichlid section, possible tankmates could be 'mellower' large cichlids, oscars, severums, depending on tank dimensions maybe peacock bass. Though individual personalities can influence stocking with cichlids a lot, so you might have a super aggressive oscar, but I think generally an oscar should be OK. Some people do keep the more aggressive cichlids (such as red devils) with CK's, I wouldn't personally do it but if you have a back up option you may be willing to risk it in a large tank.