I've been through that situation twice, with mixed results.
First time was a smaller tank (30g) and only 4 discus adding a 5th (all 2-3"). Went great - fish acted like they were long lost pals waiting to reunite.
Second time was a bigger tank (55g), 5 discus (had been relocated from the 30), adding a 6th (fish now ~4"). Didn't go so good. The new guy got chased a lot, especially at feeding times. I ended up taking him out after a short while, putting him in a isolation tank, and power feeding him for a week or 2. He got a little bigger than the others, went back in, and was able to hold his own much better. Still gets chased some, but not anymore than the others.
Wish I could give you a more concrete answer, but thats how it happened for me.