NO! I Would not reccomend putting a flowerhorn or anything like a jag or agressive cichlid in with your clown. I tried this a few months ago, thinking that, as the flowerhorn was only around the size of the CK's mouth, if not a little bigger, that it would be intimidated by the knife who would be dominant by size. Instead, the flowerhorn, with a desire to be the most dominant fish in the tank, took the clown knife's being bigger as a threat to his status and started picking fights with him mercilously (You know that thing thats in movies about prison, where they say you find the biggest, strongest, guy and beat him up to gain the respect of your peers? Cichlids, especially the more agressive SA's, think this way.). A clown knife will bite some face if it feels like it has a chance, but is not an agressive animal per se, more of a spook or a ghost. They are comparativeley thin and fragile and no match for a heavy bodied cichlid with something to prove. CK's don't want to be dominant over cichlids, so they will just hide or run away. And we all know thats no fun.
So, what I'm saying is, don't make the mistake I did, and don't try it. (oh yeah I forgot to mention that I took the FH out that same day, it would have been bad otherwise. Carlo the CK is doin fine.)
Keep a CK in a community tank, that way hes well fed

. Thats where carlo is now.