Jardinis are notorious for being aggressive. Mine was 7" when it killed a 5.5" pbass and harassed a 4.5" to near death. Had to put up a divider in the tank for the until the day i sold him. It might work out for now, but one day it might snap and kill them.
Totally agree about the Jar! They do have a nasty reputation. I only have a silver, so I can't say from experience though...
In addition, discus are known for being shy. I mean, depending on what you read, silver dollars stress discus out and they're far from aggressive. On the other hand, as long as the aro stays at the top where he belongs, maybe it could work... You just don't want stressed discus. It sucks if they don't get to full size 'cause then they never reach their bad ass potential