The issue with hermits in nano tanks is that they become far more likely to hunt snails. I believe it is usually out of lack of food that a nano provides for them. I've also seen hermits kill snails just because they can. They seem to really like cerith and Nassarius shells due to their shape.
I would remove the hermit and turbo snails. Turbos will knock stuff over, especially in a smaller tank. I also don't think Turbos are suited for nano tanks, as they usually starve.
In a 12 gallon, I would do 3-5 of each: Nassarius, Cerith, and Trochus and/or Astrea snails.
Depending on how much you feed the tank and what's in it, you might need to slightly adjust that list.
If crabs interest you, try a Pom Pom crab or Emerald crab. An anemone crab might work as well (they don't need an anemone).