Hi Steffen and welcome to SimplyDiscus! I'll try to be gentle here, but you seem to have made many of the mistakes that folks new to discus do. Many of us started like you, hoping to raise these beautiful fish, but you've got lots working against you. So, where to start. Yes, your fish look stunted and in poor health. It is unlikely that they will recover and grow into thriving adults. Whether this was caused before you got them or after is hard to say. Raising young discus (juvies) usually requires almost daily large water changes, and a bare bottom tank environment, with no other fish, to keep it clean and minimize the potential for pathogens to grow in the tank. For that reason, beginners are usually encouraged to start with groups of 6 or more near adult fish (more expensive) until they get a handle on the maintenance required. Your fish in the planted community tank with gravel has many of the negatives working against it. I encourage you to read up on the "sticky threads" in the beginner section, including this video link I'm including.
http://forum.simplydiscus.com/showth...TCH-THIS-VIDEO!
I'm wondering if your two new fish are the proven pair you've posted the pic of. The simplest recommendation for them would be for you to start over with a cycled bare bottom tank just for the (don't mix in your others), and go from there. Please read up and ask more questions when you are ready. There are many here ready to help.