In addition to the nassarius snails, I would get a dozen astraea snails. The nassarius snails are good for your sandbed, the astraea snails are good for your glass.
If you buy too many cleaners in the beginning, they will starve to death and die. A better approach is to gradually add them over time, as you need them, to ensure your tank can feed them.
Other good snails are nerite, trochus and Mexican turbos, but I wouldn't get more than two Mexican turbos for your tank, as they get really large and eat a lot.
I personally do not like hermit crabs because they kill snails, so you constantly have to be replacing snails. If you want a handful of hermit crabs, you can get a dozen or so of those guys to start too.
After the initial batch of snails are added, you may find you need to add more. Eventually, I'd get two serpent or brittle stars for a 100 gallon tank, as well as three or four emerald crabs. Later on down the road is when you'd want to add shrimp, such as cleaner shrimp or peppermint shrimp.