the general thought for discus is they shoudl be kept in groups 4-5 is considerd the minimum group.
you can house 4-5 discus in a 55.
(sorry not mauch help on the jd's)
typically you need to feed the discus and treat them special to get them to that size..many are bred to be just this. I have seen them( I have several friends who are discus breeders)I wouldn't house Discus in anything smaller than a 70 gallon. I would want that extra 6 inches from front to back. They can get to be the size of a dinner plate. You don't want that in a tank that is only 12 inches from front to back.
As for Dempsies, the mailes will be bigger and a little more colorful than the fems.