By medium size I'm assuming you mean 3-4" in total length? If so, they need 4-5 feedings a day if they are going to grow to normal adult sizes.
You don't need to just feed one food. Discus will eat flake food (buy one made for discus) and pellets without any problems. By feeding them the flake foods during the day, you can then do just one nighttime feeding of beefheart or some other high protein frozen food and then do a WC with a good vacuuming.
Doing it that way is what I've found to be the easiest with a smaller tank (mine is 54 and pentagon shaped, so not easy to vacuum).
Be sure to buy a pre-filter for your emperor intake tube when you buy the hydor sponge filter. The only other option is to turn off the filter each time you feed the beefheart and that's just a pain in the patoot!
You can also remove any mechanical filtration media from the emperor filter and fill those spots with more bio media since the pre-filter will do most of the mechanical filtration for you.
Buy Discus Trace Elements for your RO water. Oh wait, you didn't mention whether or not you absolutely must use RO. Is there something about your tap water that's a problem?
For vacuuming I use a Python type syphon and dig into the gravel around the plants, then swish it in a circle above the plants in order to force any hidden food or poop up into the water column where I can then syphon it out. There are other vacuums you can use if you don't use a standard syphon system for water changes. I know Eheim sells one called the Sludge Extractor. And I have a small battery operated one that I use on my mini fry tanks.