Check out his article on my web site about a commonly found hermit that you want to avoid.

http://coralreefbazaar.com/Tips_Article_Read.php?ID=25

Hermits that are good for your tank are blue-leg and red-legged hermits. The red-legs get much larger than the blues. They both grow pretty quickly so you will need old empty shells of various sizes for them as they get bigger. If they cannot find a suitable shell, they will take out your snails to make a home.