It usually isn’t necessary to enforce a strict quarantine on snails and most other invertebrates that are going into a seahorse tank. For one thing, invertebrates in general are not susceptible to the same pathogens and parasites that plague seahorses and other marine fishes. If they were carrying any of the parasites that could bother seahorses, it would be as hitchhikers, and that’s unlikely because those same parasites normally cannot survive long without a suitable fish host. So there is relatively little danger of introducing seahorse parasites via the snails we use as sanitation engineers.
Secondly, snails in general cannot tolerate the usual prophylactic measures we apply to marine fishes when we quarantine them. For example, many types of snails cannot withstand hyposalinity let alone a freshwater dip. Nor do they tolerate the usual chemi-therapeutic agents we normally use to cleanse quarantined fish of parasites, such as formalin, malachite green, copper sulfate, dylox, since the etc.. So there would be very little we could do to treat snails or other invertebrates prophylactically or preventively even if we quarantined them indefinitely.
So I normally don’t quarantine the snails for my cleanup crew. Rather, I screen them visually to avoid snails (or micro-hermit crabs) with telltale fuzzy pink patches on their shells indicating hydroids or that may be carrying nuisance algae. If in doubt, I’ll use a clean toothbrush to scrub their shells clean and then carefully acclimate them to the aquarium.
If you want to hold them in your hospital tank for a week or so first to make sure the snails are healthy and disease-free (free of diseases or parasites specific to snails, that is), that’s fine as long as you can feed them during their stay in the quarantine tank. (Sheet algae weighted down beneath a small piece of live rock or coral rubble will suffice for this.)
An assortment of the recommended snails for your cleanup crew should generally have no interest at all in the colorful coralline algae that encrusts live rock. The Nassarius snails, for instance, are primarily detritivores that will clean up the meatier leftovers in the tank, and the herbivorous snails that make good sanitation engineers and aquarium janitors typically specialize in eating unicellular microalgae and diatoms rather than decorative plants. None of my snails have ever shown any interest in the Caulerpa macroalgae in my seahorse tanks. In short, the snails suggested as sanitation engineers dine on the film of diatoms and microalgae and should not bother coralline or most types of macroalgae.
As for the number of snails, but reef keepers will often employ up to 1-2 aquarium janitors per gallon, but the average seahorse setup can get by with quite a few less sanitation engineers. In a seahorse setup with live rock perhaps one snail for every 3 gallons would be more appropriate.
Best of luck with your cleanup crew, Nan!