Hi Cyrus and welcome .
Its always safer to start with more and take out the least desirable along the way than the other way around . Adding fish every couple of months is risky thing to do.
Regarding your dillemas , yes you can keep 2 discus i n a QT for a month or two together without any aggression issues .
Choose size and age to match your current batch , meaning 3-4 inch discus .
Keep the new 2 discus for 4 weeks by them selves and if they behave and eat normally add only 1 test discus along with them for additional 2-3 weeks testing period.
If they are all happy together , then you put them with their rest of your discus .