These are the current clean-up critters: one astrea & 5 blue legs hermits, and a coral banded shrimp.
I always recommend a variety of snails, as I think you get the best algae competition. I think adding more astrea snails, some ceriths, trochus, nerite, and maybe one or two turbo snails will be a nice combination for you. Since you don't have a substrate, then I'd stay away from the nassarius. Also, as was previously stated, you don't want to go overboard either because once the food source is gone, then the snails will starve. When they die, they will be releasing all the nutrients they used when eating all the algae. Just be sure to remember all of these things poop, too, so your husbandry will have to step up.
Keep working on getting the nitrates down. I don't remember, are you using RO/DI water? Have you tested your premixed salt water for nitrates prior to doing a water change?