I have one, one of my favorites by far. If he got stressed out enough he absolutely could release a toxin into the tank and kill everything including the live rock. Just because it rarely happens doesn't mean it wont or can't.
I got lucky with a small one that I had (before the one I've got now) that died and thankfully didn't release any toxin. From what I understand they only release it under extreme stress, sometimes in the bag of water during shipping, during transfers, being bullied by other fish or exposure to air. Basically make sure the lfs bag's it under the water and doesn't expose it to air and when transferring it to your tank totally dilute the lfs store water with your own and use a plastic container to scoop it out with (so it's in water) to avoid air exposure and release of toxin.
From what I understand they wont eat coral but sometimes will eat snails or shrimp.
Very funny fish to watch worth the risk IMO. They do need low flow because they have a hard time swimming around, and peaceful non nippy tank mates.
You also have to watch how your rock work is laid out. They have been know to get stuck in the rock (wedge themselves into a spot they thought they could fit but couldn't) and die from being unable to free themselves.
Some good info
http://marinebio.org/species.asp?id=1463
http://animal-world.com/encyclo/mari...rs/cowfish.php