Not sure about the crabs since I have not bought any in awhile but for snails, I try to get a mixture of Cerith and Nassarius. I love the Nassarius since they work the sand bed but Cerith do more up top. They are not big either so they dont knock stuff over like turbos. Plus they can right themselves.
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,