I've never had a problem with hermits, but I know they tend to fight over shells, I've seen this. Some people claim that they attack and kill snails for thier shells, but I figure if you go to a craft store and buy a bag of shells (which I did) and drop some empty ones in there, they wont fight or kill your snails.
The RO water will help with keeping your sand cleaner, there's alot of stuff in tap that isn't good for the tank. Obviously you know that cuz you just switched to RO.
Nassarius Snails I found where very hard to find, unless you want to pay $1.50 per snail and then deal with the shipping. I got lucky and a guy in my reef club here, ordered 50 and got 150 so he sold me 10 for $10, great deal. They are very fast, and burrow in the SB, when they "smell" food they pop out of the SB, and go straight at it. I don't think I would have a reef tank without them now, they are too fun to watch.