I'd say it's definately unsuitable for your tank and not just because your tank is too small for him and some of your other fish. The other tankmates are pretty aggressive fish when it comes to eating and if your clown is anything like other GSM clowns they can be pretty aggressive. You are aware that if bullied, stressed or ill, that your cowfish will exude a toxin that could wipe out your whole tank including the cowfish?
My scentaments entirely. I just dont think that this fish is suited overall for your aquarium. I base this on the size that the fish can be and the potential damage caused from the toxin in a small environment.