Although I am no expert, I am sure the experts are going to say...
"You need to have a minimum of 6 Discus in the group, so that the agression is spread more evenly amongst the group"
Good News: With a 150 gallon tank, you have room for quite a few mroe Discus.
I am sure they will also mention with such a low kH you run the risk of pH dropping quickly due to the low buffering ability of RO water.