By pristine, I mean zero ammonia and nitrites and 10ppm of nitrate. Temp at 84 degrees and the pH is best at 6.6 although not that necessary. You first need to have sucessfully kept the discus before adding the aro. Discus need to be kept in groups of 5 or 6. Best to heavily research discus keeping. Once you have discus that are thriving, keeping with and aro will be easier.
Black aros are not more or less aggressive then silver aros. With large aros there is always a chance of a discus ending up in the aros mouth, sometimes it is even accidently.
Realize that to keep the water params where they need to be for discus means alot of W/Cs. So in a monster tank, it'll need to be either understocked, planted or have a drip system to help with the water params.