Are the rocks next to it fairly stable?
Yes, they are very large and stable, at least best as I can tell.
How about that long horn cow,has he/she ever nipped at anything?
It has never bothered anything that I could tell, except some zooanthids that I put at its end of the tank, which it ate all of. One time it kept picking up a hermit crab by the shell, but it never got in. I expect it has eaten some small inverts, but that's life an a working ecosystem.
Also, if stressed,will the toxin affect inverts/anemones
or just fish(God forbid!!!)
Before I got this one, and its predecessor that I also had for a few years, I searched all though RC looking for anyone with first or even second hand exdperience with such a scenario,, and I could not find a single person that had a toxin release event. I've still not encountered such a person or thread. That doesn't meant it cannot happen, but I suspect the risks are overblown.
That said, it is something the whole family is aware of, and we keep an eye on it. IT is also by far our favorite fish. All the other fish scare easily, while it swims over and interacts with us anytime we get near the tank. I typically hand feed him chunks of food, although I've become a little more wary of his mouth as he has gotten bigger.
IMO, the biggest issue with them is the size they get too. They get quite bit.