I wouldn't put any echinoderms in a pico. Hermits do ok as long as they're small species and they arn't over-crowded. Some are ok, just keep in mind that grazing snails may deplete the available food in the tank quickly and risk starving. Nerites are good pico-sized snails, although they're definitely not compatible with most hermits because their shells seem to be the top housing pick for a number of common hermit species.