Actually I've been asking about sand sifters and the LFS's seem to recommend gobies as well, especially Sleeper Gobies. However, yeah they can be a bit picky about what they eat but supposedly it's not too difficult to train them to eat other foods. And yes, they do normally do much better if you get them as a pair. They seem to pair up quite early in their lives and a dealer should be able to supply a pair.
Here are a couple of good articles on reef Gobies:
The Care and Feeding of Sand Sifting Gobies
The Substrate Sifting Gobies: Fishes that Earn Their Keep
Other choices for sand stirrers are Nassarius Snails and Cerith Snails which both like burying themselves in the sand. Sea Cucumbers are also good sand stirrers but get quite large and may make live rock arrangements unstable or fall. Some sea stars will also stir up sand. Some conches are also good sand stirrers but can get a bit big and clumsy.
Here is a (commercial) listing of some good custodial crew that could also sift sand:
BYO Reef Cleaners
Thanks to Harvey for answering some of my questions on the subject earlier. Hope that helps.
