NO. You may ABSOLUTELY not add more fish to this tank! Just because the discus are small now, do you think they will never grow? Keeping them in a tank that small will stunt their growth and is very, very cruel.
We all gave our advice. You are choosing not to take it. If you cannot have a bigger tank, 55 gallons at least, then your discus will never be healthy. We are not saying these things to "dissapoint" you, but because we don't want to see your fish die. You are choosing not to listen, and there is nothing we can do about that. Best of luck to you, but I fear that the worst will happen.
Kristina