Snails are fine with dwarf seahorses but I wouldn’t trust the zebra Hawaiian hermit crabs with them. If you want to try micro-hermit crabs with Pixies (Hippocampus zosterae), consider the herbivorous Scarlet Reef Hermit Crab (Paguristes cadenati) instead.
Nassarius snails and Scarlet Reef hermit crabs (Paguristes cadenati) are the cornerstones of the clean-up crew in my Pixie tanks. The Scarlet Reef micro-hermits are colorful and interesting in their own right, and these harmless herbivores are the only hermit crabs I trust with my dwarf seahorses. A half dozen of the colorful Scarlet Reef crabs make nice additions for a dwarf seahorse tank and clean up algae and detritus, and the Nassarius snails are very active, efficient scavengers that handle the meatier leftovers. Between the two of them, I find they do a good job as aquarium janitors and help keep my Pixie tanks clean.
Best of luck with your new Pixie tank, seahorsegeek!