I had a small nano like that and I had a blue chromis and a coral banded shrimp. They both kept each other at bay for years, and tolerated the fluctuating parameters that a small nano could have. Both were very active.
If not that - a pair of two spot gobies or a watchman goby/shrimp pair.
If not that - a pair of two spot gobies or a watchman goby/shrimp pair.