Okay about your 1 to every 1000 SF comment.
You are wrong, I see that you have never been to the Florida Keys. If you have, go below one of the seven mile bridges. You can easily find over 5,000 in a day. Hermit crabs are actually really good for a tank, but I would suggest scarlet, not red legs. I have over 75 scarlets in my larger reef tank and have never seen them kill a snail. Now my blue legs in my agressive tank, I've never seen them eat a live snail, but dead or turned over types are another story. But as long as you make sure they get enough food they are fine.
Since you will ask, put a little seaweed on the floor of the tank feeds the little devils just fine. Also, snails only live about a year or two in a tank because they are at the end of their live cycle when we get them anyways. I have had snails reproduce in my reef tanks many times. Now I have snails from very small to very large and no the blue legs are no making dinner out of the small ones either..
So get the usual amounts of hermits and have fun watching them... they actually due serve an important role.