If you are only gonna keep inverts, your rocks and sand will help you do the biological fitration. Just have to make sure you got enough flow in your tank. Yup cleaner clams will help keeping good of your water quality. I have a friend who buys clams from superstore and just chucks them in his sump. I also have a small nano invert-only tank. Only started about 3 weeks ago and so far I have a pair of harlequin shrimps, 5 sexy shrimps, a procelein crab, 2 astrea snails, and some feather dusters in there. For keeping clean of your tank, hermits tend to do a better job on hair algae and snails more on film like algae. Algae/diatoms on the sandbed you can go with cerith snails or a sand sifting starfish. Hope this would give you some brief ideas.
