Its not that you are overstocked, its just that Discus have really specific needs and if these are not met they tend to fade away quickly - generally they stop eating, go black and die.
Most people agree its best to keep them in groups and to do this with a group of 6 inch fish you need a tank bigger than yours at present.
Jacko above here said 75g which I agree with though have seen people have success with 50/55 gallon tanks.
Discus need higher temperatures than most other fish, which helps with their respiration system, growth and metabolism they also benefit from low Ph and in most specimens this is a necessity.
They are also very sensitive to Nitrate in the water and most community tanks can not give these levels required - nor can the other fish you mentioned thrive in Discus conditions - mainly the high temperature.
With Discus its not like stocking a general community tank - ie building a community based around (the flawed) inch per gallon rule. In a Discus tank its about keeping the Discus alive and healthy which means trying to provide the requirements I mentioned above here.
Discus are not community fish