The problems I've had with the nanos are mainly temperature, the rate of change in water chemistry, and the limited space. If you've got things out of balance or are dosing incorrectly it is incredibly easy to have something test off the charts one day and be nearly non-existent the next.
A coral banded will be fine in there, but they won't tolerate the presence of other shrimp. A few people have reported problems of the shrimp actually attacking fish, but it is pretty rare. It will cost slightly more and be harder to find, but if possible you should go for a blue coral banded shrimp. They stay considerably smaller and have a very nice blue color to their body.
Serpent stars are for the most part too large as PD said. There are some species that stay very small and make excellent additions to the clean up crew in a nano. They are harder to find, but they make a great addition if you can get them.
Snails are very different between species. A lot of species are not suitable in a nano reef tank. Some get too large, some are predatory, and some don't tend to do well in reef tank temperatures.
Some snails I like are Astreas for general hair algae grazing. Nassarius(the smaller species) are great for detritus clean up in the sandbed. Stomatella varia are awsome all purpose algae grazers that can often be found as hitch hikers on live rock. Strombus maculatus work well for eating algae. I find that they breed a little too quick for my liking. I'm constantly cleaning egg cases off the glass. Cerith snails will also eat algae, but they lay long strings of eggs on the glass.
There are a few snails that are commonly seen that should be avoided in my opinion. Bumblebee snails are really cute, but they are sandbed infauna predators and are counterproductive in reef tanks. Nearly all of the cowries and whelks are also predatory. Most conches get far too large for nanos. Margarita snails come from the cooler waters of the Gulf of Mexico in most cases and do not live long in 80* reef tanks.
When it comes to crabs I try to avoid them. All crabs are opportunistic omnivores. So if that emerald mithrax crab sees a small sleeping fish or shrimp or even a tasty coral it won't think twice about making a meal out of it. It isn't a common occurrence, but when you make the tank smaller the odds of an unfortunate meeting rise.
The dwarf hermits don't pose a threat to fish and shrimp as they are much smaller, but they are capable of putting the hurt on a snail population. I've got a single hermit in my 12 gallon and he has claimed a handful of snails. He is cute enough to be worth the trouble, but I don't think I'd keep more than a few hermits with snails. Unless of course the tank was very large.
