You've got plenty more room! Amount of corals is dictated by what kinds, some need some space or they will sting their neighbors. Most mushrooms can co-exist right next to other mushrooms. They are an anemone though, and they can sting other types of corals, like Zoanthids ( like your green ones ).
When you have corals encroaching each other, inhibiting each others growth, then you have too many corals. Till then, if allowing for some growth or a safety zone around more potent ones, you can put corals wherever you have space.
Good luck with your further researching and stocking!