cowfish aren't as likely to have a toxin as a regular boxfish. I think the only documented occassion this has happened was written by Bob Fenner and I haven't heard any since or even prior. If you do a search on these I think you will find the "toxin" release is an over-exaggeration for the most part bordering on myth which gets perpetuated through the internet by people who haven't had experience wiht them but have heard stories of this. Like many things in this hobby sadly enough. "If I heard about it on the internet it must be true".
I had two get stung by a massive carpet anemone with one being killed and the othert surviving. no toxins. the survivor was stung a second time and survived and still no release. Both of these cowfish were over 5" long. There used to be a cowfish group on MSN and you will hear of many whom have had these fish pass away with very few/no instances of toxins. For all the cows I have seen sold in this area alone I have yet to hear of anyone with a first hand toxin experience.
FWIW
I will add that yes there are boxfish that do have this potential and I have seen instances of this happeneing. But with cowfish I have never seen or even heard of this happening other than Bob Fenners one report.
I believe that these fish have been lumped in with others by mistake and it has stuck. Regretfully. They are some of the best fish to keep IMO IME. But they do grow fast and large so you WILL ned a big tank for them. They make a big mess and will taste and munch on anything that suddenly is interesting to them.