Yes, sir, the Hawaiian Dwarf Hermit Crabs (Calcinus laevimanus), also known as left-handed hermits or Hawaiian zebra hermits, are compatible with seahorses and should get along well with the Mexican red-leg hermits you already have.
The Hawaiian Dwarf Hermit Crabs stayed fairly small (less than 1.5 inches when fully grown) and are useful aquarium janitors. They will clean up leftovers but are primarily herbivorous and will eat all types of algae, including nuisance algae such as red slime algae or cyanobacteria. If there isn’t a good growth of algae in your aquarium, you might want to consider supplementing their diet of these hermit crabs with dried algae (e.g., Nori, sheet algae, or Spirulina flakes).
If your seahorse tank is going to include microhermit crabs, I like a combination of Dwarf Blue-leg (Clibanarius tricolor), Hawaiian Left-handed Or Dwarf Hermits (Calcinus laevimanus), Mexican Red Legged Hermits (Clibanarius digueti) and above all, Scarlet Reef hermit crabs (Paguristes cadenati), which are my personal favorites.
The Scarlet Reef Hermit Crab (Paguristes cadenati) is a colorful micro-hermit that’s a harmless herbivore. So cannibalism isn’t a concern at all for these fellows, nor are they likely to develop a taste for escargot. As hermits go, most of the time the Scarlet Reefs are perfect little gentleman and attractive to boot. I even use them in my dwarf seahorse tanks. Best of all, they eat all kinds of algae, including nuisance algae such as red, green and brown slimes, as well as green hair algae.
If you’re going to have any hermits, stick with species like the above, which are known as micro hermits because they start out tiny and stay small. Avoid Anomura species of hermit crabs no matter how small they are, however, because they will kill Astraea snails to obtain their shells.
Best of luck with your cleanup crew, sir!