I'd agree with Kevin. Trochus, though somewhat more pricey, are much harder workers, and are able to "right" themselves if tipped onto their backs. Astreas cannot.
I'd actually look for more of a "mix" - smaller numbers of each, but trochus, margarita, nerite, cerith, nassarius ... etec.
I also would not do the Emerald Crabs unless you have a Valonia (bubble algae) problem.
I'd consider 2 Peppermints, and possibly 2 Cleaners.
, check out ReefTopia.com. I've always found good prices there, and have had very good experiences with them.