they are omnivores, the Cowfish should be offered a variety of meaty and vegetable foods. They are slow eaters, and should not be housed with aggressive eaters.
I've trained my little 2" cow fish to different foods in my sump and he's been in there for the last 6 months. There's no way he'll make it in the main tank. He eats a lot, and only 1 pellet at a time. I calculate that he would need to be 4 to 6" in size to compete in the main tank and I guess that's never going to happen.