I love trochus snails. They are by far my favorite snails. Just make sure you don't have hermit carbs also, because they will murder your trochus snails. A handful of nice sized trochus will be very helpful in a 20g. And get a few nessarius snails to help clean/aerate your sandbed.
Reefcleaners are great guys and they give you a ton of value for your money. But they only have ceriths and nerites, and I am not a big fan of either. Ceriths don't seem that effective to me and the nerites seem like they are out of the water as much as they are in it..