Don't trust the hydrometer, a refractometer is the way to go. Hydrometers can be way off.
Good start with the tank, you have a rock wall, consider making a pillar or tunnel, it's nice seeing fish swing around and in the rockwork. I think you're on the right path with small fish, there are a lot of really cool small gobies that can make nice additions, plus their bioload is miniscule. I like the highfig goby (you can pair w/ pistol shrimp), striped gobie, clown goby (i.e. 'gumdrop') and blue cleaner goby to name a few.