Aro and discus is not a good combination. It is almost imposible.
Experience Flashback:
I started with 1 small xback and 10 or so sub-adult discus. They did well till the aro hit about 12". Despite all the driftwood and hiding places I provided the discus they will eventually deteriorate from stress. A few were bitten to death. I decided to experiment thinking that having 7 aros will desperse the aggession among themselves (aros) and leave the discus alone. And thinking that large number of discus will make them feel more secure, I have about 40+ discus ranging from 6" to 3". The 6 aros grew up with 5 adult (6") discus. When aros were about 12" I put them together in my new 8' tank. worked well for about 7 months or so then the discus started dying off one by one from stress. Several were bitten to death again. And as before I provided the discus with ample hiding spots. When lights are on the aros looks so innocent as if nothing goes on in the community. But once the lights are off, thats when all the carnage happens. Usually in the early morning when it is still a bit dark. I eventually took out all the discus. At one point the aros will be fighting with each other and those fast movements adds stress to the discus. In my opinion it will be easier to have a 2 aro comm than an aro-discus com. The only way I can see a peacefull aro-discus com is to have an aro who is totally not aggressive at all and comm him with the discus. It is hard to find an aro that is not aggressive but you might have one in your community there. It only takes one aggressive aro for the discus tankmates to go downhill. Once aggression begins identify the culprit and take him out until hopefully you'll be left with a few or one non-discus hating aro! Good luck!