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6 scarlet hermit crabs, 15 red leg hermit crabs, 20 margarita snails and 5 turbo snails
For 15 gallons you could use half of this, but this will depend on factors such as amount of rockwork, amount of algae (the food source,) etc. Personally, I have found a smaller clean up crew to be adequate for my 30 gallon aquarium.
I would say maybe add 1 more hermit for now, 10 is too many IMO. I would suggest adding maybe another snail also. As your tank matures and produces more algae you can start adding more inverts accordingly so they don't starve.