I have a 26g system, 4 is where I am stopping.
There's some great basslets other than the Royal Gramma, that especially shine in smaller systems with peaceful tank mates ( though they are a little pricier- Gold Assessor, Swissguard, Cave, etc.). I have a Gold Assessor and he spends most of the day swimming out in the open as soon as it gets light in the room.
Cryptic Wrasse, similar in size to the Banded Possum mentioned above. Zooms around the tank looking for food. Super peaceful.
Yasha Goby/pistol shrimp combo.
Per above posts, Clownfish are generally aggressive (to cohabitants and likely you). They do have great personalities and if you are set on them, make sure they are the last fish in.