Discus are also very sensitive to water quality, pH, kh, gh, temp etc. They are probably the hardest tropical fish to keep, and as mentioned, they like to be kept in groups. They are very docile and they will literally get bullied to death in your tank, and not just by the JD. Best to avoid discus unless your going to set up a discus community tank with similar docile community fish. Please don't add your discus, you will end up going through this again.
You can probably add more fish if you add more caves, even if only temporarily. I've found that rearranging the tank before adding in new fish can help as it effectively 'resets' the territories. Also, bringing down the temp a bit can reduce aggression in cichlids.
Don't give up hope. I've kept large aggressive American cichlids for years and I've not had that much trouble when adding new fish. Just be sensible. Don't add a 2" fish with your 10" fish and vice versa. Maybe try to find a larger aro. They often get sold secondhand due to outgrowing tanks, which you also need to bare in mind with your tank. They need a good turning circle or they can become deformed. I'm about to upgrade to a 8x2.5x2.5 pretty much just for my Arowana.
+1
Yuki Rihwa
O
Oddballs
Wise words.