Here is a good "list of inhabitants" for a Biocube 8:

5 astraea snails
5 small nassarius snails (keep the sand clean)
2 peppermint shrimp (cleaners and pest eaters)
1 emerald crab (good for nuisance algae)

0 hermits (tend to kill snails, even if extra shells are available)
0 turbos (these will not have enough to eat in such a small tank)
0 starfish (need a much larger sand bed to survive)

1-2 small fish

~these are just some ideas that have worked for me, you may end up with something else of course though...