yes i have had a blue jaw trigger and a long horn cow in a 180gal. they seemed to get along but of course the blue jaw is one of the calmest triggers. but i wouldnt suggest any other trigger. the only problem that i can foresee is getting the cow to eat they typicaly not very agressive eaters and triggers are. also the longhorn cowfish needs a very large tank.
Yeah the tank is going to have to be large (A. Cowfish need biiig tanks and B. There needs to be enough space so the trigger doesnt harass the cowfish.) A less aggressive trigger is a must...cowfish are very sensitive to stress, so bullying from an aggressive trigger will be very bad.
When I first got into saltwater, my girlfriend wanted one, and the moron at the LFS said that my triggers couldnt hurt it because its body was too tough. I had 6 baby triggers, and it was a massacre. I now have a baby cowfish in my community tank with a foxface, two clowns, a hippo tang (I know... Pathetic!)(Wife, this time),flame angel, and a purple lobster. He's awesome. He eats out of my hand, and loves all of his little buddies.
The LFS has a 7" cowfish in their display tank (300 or 350g) with a bunch of fish, including a Clown Trigger that is around 8" or so.. I never saw the clown harass any of the other fish in the tank, including the cowfish. However.. you never know.. which brings me to the next question..
Has anyone had a total loss of their stock because of a dead cowfish? I mean, I have a bunch of angels, soem triggers and groupers, and I hand feed my P.Puffer and Sunset butterfly to ensure they get enough food, so I really wouldn't mind the extra time with feeding him.. but what happens when they die? Do you really lose everything? I am too scared to try.. I have like 9 angels, including a chrysurus.. and they are all doing well including eating pellets until they are FAT. Would really hate to mess with that at the moment... but Cowfish is right up there with clown triggers and pinecone fish as my 'favs'.