I know this thread is ancient history, but when you google Long Horn Cowfish care, this is the most recent one that comes up.
I just wanted to add my two cents as I keep hearing the "they're poisonous!!!" thing too. I've had my guy now since late february in a high flow SPS dominant, 275 gallon reef.
He is easily every one's favourite fish. A notorious beggar who will sit in the middle of the tank staring directly at you while you eat dinner in the dining room. Not the best swimmer in the world so I have the pumps go to 50% power at night, but seems to cope with 2 MP60s, 2 MP40s and a 2000GPH return pump pretty well. Was eating out of my hand his first day in QT.
So far he hasn't bothered a single coral, not SPS, LPS, or zoas. The closest I would say he comes to anything that could resemble picking on a coral is that he's figured out that bits of mysis and brine shrimp get caught by in the tentacles of the longer polyp acros and hydnophoras so after the main feeding frenzy over, he'll hover over them picking off any bit of food he can spot, which causes the corals to close up for a while. I've had no issues with skin poison and don't expect to.
We're moving so something is going to have to happen with this tank soon, but he's cool enough that I'd actually consider setting up a 250+ gallon aquarium just for him.