i kept one for a number of years (i was misinformed about them being reef safe). they get huge (bigger than a baseball) and will eat any fish they can catch (being true to form as opportunistic omnivores).
i finally tossed it in with a 2.5' snowflake eel, thinking both would be safe, but the eel actually ate it, which was fine with me, as it had been quite destructive over the years no matter where i put it.
personally, i'd worry about it getting ahold of your cowfish...