For snails, i like ceriths for this. They don't cover alot of ground individually but they have nice long snouts to get in crevices and as a bonus they burrow and turn over sand.
Depends on if you have your zoas high on rocks or low in a "garden". I have one small conch that is my zoa cleaner. Anytime GHA starts growing on a zoa frag, I just move the frag onto the sand bed in front of the conch and the frag is clean in the AM.