For a nano maybe 3-5 Astrea snails,3-5 small nassarius snails,1-2 ceriths and a queen conch would be my clean up crew in a nano.
I would go with only 2-3 small fish.
Dartfish-Purple Firefish,Red Firefish
Gobies-Yellow Watchmen,Diamond watchmen goby come to mine.Stick with the smaller species.Some grow very large.
Clownfish-I would stick with ocellaris(false percs) clowns or the black/white ocellaris clowns since they are the smallest of clowns.
There are many more so research what you like.