It's coming along.
You don't need a certain amount of sand like someone posted before (you could go without any sand), but what you have is fine. I agree you should add more rock though, and it doesn't have to be live, it can be dry. Just saying damsels are known for being territorial so watch out for aggression when you add a new fish. What are your parameters? If you don't have a test kit already, I'd say it's a necessity so that you can know when to add another fish, how to fix a problem, etc. Don't get a new fish until you test your water and know that the cycle is over (0 ammonia, 0 nitrite, low level of nitrate). I'd think that the damsel (can be extremely hardy) and apparently the gobies are surviving in levels of ammonia and nitrite which are very toxic. A new addition wouldn't be so "lucky".