recently got a great specimen from CF. wide selection of them when I was there. so far no problems towards my other fish, you might have trouble with shrimps and other invertebrates.
Most probably it's no. 3. I saw my hermit crab limbs lying on the ground one day but it was just walking around near to it. But you still probably have a mantis/pistol in your tank, just that it hasn't killed your hermit.. yet..
Ahaha my 3inch mantis has escaped into my main tank countless times. usually I just put some food at the top and he will come swimming out and I just net him.. quite easy