An appropriate combination of small gobies and blennies, and other little fish, will probably work just great.
Although their impact is fairly small, hermits, snails, shrimp and the like, still contribute to the bioload. Yes they eat up leftover food and stuff.. snails and hermits eat algae... but they also all poop just like the fish do! Haha. I cant say anything for the quantities... but im sure that a few hermits poop just as much as 1 fish might.
A member on the site here, Bev, did a photo or video record of her tank that only had invertebrates in it, (tanks were live rock, corals, and assorted snails/crabs I think?). Anyways, the tanks were barebottom, and she was still syphoning out a nice pile of detritus on a regular basis.
Anyways, not saying the critters dont help. They certainly do, by making algaes and leftovers easier for smaller inverts, bacteria/microbes etc... to 'process', but keep in mind they still transfer the energy back 'out'.
I guess its not a big deal with just a few... but I dont like folks thinking they are entirely inconsequential, haha.
Cheers,
Chris