Hi Crew,
I know of two "red" hermits: scarlet reef and red-legged hermits. The scarlet reef are the more common of the two. From my experiences, scarlet reef are often harmless to corals, but not always. I eventually saw 3 of mine picking on live coral tissue, leaving holes. One of these was really big. Along this line, I've heard that the bigger they are, the more trouble they are for eating perfectly healthy tankmates (corals mostly).
Red legged hermits I've read are more troublesome right off the bat.
The others you probably know about are blue-legged hermits, which are most often thought of as the most problematic of all hermits. Some have no problems with them, so personality may play into it, but most seem to have found them to be very aggressive (killing snails, eating corals, etc).
I've never heard of any of these hermits attacking fish, but I suppose anything is possible.
A decent number? Hmm, this might generate more of a debate. I would say if you have corals, you can do better without hermits (with say peppermint shrimp and starfish as cleaners instead). But if you really want them, I would say for the average stocked reef tank, maybe 1 per 10 gallons. If they are quite small, you could double the amount (1 per 5 gal).
kris
