In my 20 long I have done well with the typical blue leg hermits and emeral crabs. Haven't had much luck with stars. I think there are too many variables in a nano for most stars. I have done extremely well with my bubble tip anemone (I know there will be a hundred people saying that you shouldn't keep anemones in a nano or possibly any tank for that matter) but they say that you are "successful" with an anemone if it reaches on year old. Well, mine will have it's one year next month
I also highly recommend a sea cucumber. They do a good job at cleaning the sand.