I've heard good things about emerald crabs.... they're typically docile and not a problem in the tank (though if they run outta food who knows)
I like the way the tank looks but how many lbs of LR do you have? You may want some more base rock to fill out the tank more (I say base because you have LR and the base is cheaper and will quickly become LR). That is entirely up to you though.
Get some corals in there, they add so much more personality!
What type of snails, I like a mix of tubro/nassarius. There are other types but the turbos (mexican or even Astrea) manage the LR and glass, and I like Nassarius for the sand. Plus the nass are a ball to watch rise outta the sand when its feeding time.