one thing to remember when sizing your crew is how much work is there to be done. Does your tank and its critters generate a lot of waste or only a little?
This is a broad answer but many of the invertebrates you'll see while researching. There is a lot to look for...my favorite hermit at least being the zebra foot hermit crab (has black and white striped legs and claws).
In my 20g I have a great CUC.
- 1 banded serpent starfish
- 1 cleaner shrimp
- 1 turbo snail <--- these are REALLY great algae eaters.
- 2. nassarius snails
- About 5 hermit crabs
- And a bicolor blenny.
I would definitely recommend the turbo snail, nassarius snails, hermits, and the starfish.