Mine has maxed out at about 11", and yes they are very very fast and yes they keep themselves to the top 6" of water, most of the time. Mine is also with algae eaters, dennison barbs and clown loach, smallish ones. It has never showed any signs of aggression to these smaller fish.
Like I said, I have never kept discus but i'm betting that these two unlikely bed partners will be fine together. I'd be interested in anyone else's experience.