If you got a big old tomato clown, you could trust him to defend an anemone from a fat cowfish. Unfortunately, tomato clowns are mean, and very territorial. Your other fish would not like a tomato in the tank LOL.
I think a coral beauty would be a much better bet. They don't stop swimming and are AWESOME. And they cost only 22 bucks. You need to feed them plenty of green foods.
You could also consider a percula clown, a very social fish.
Warning: Cowfish are extremely toxic when they die. Your other fish will get poisoned when the moo moo passes on. I think you should rope the cow and put it in a separate tank or back to the LFS.