Ive kept apisogramma cacatuoides and rams with discus very well together. They seem to completely ignore each other in fact. Even when the male ram is frantically chasing the females around the tank the discus how no sign of notice. I have also introduced a pretty small female ram into the tank after it was established with no problems what so ever.
In my opinion, swarf cichlids such as apistogrammas and blue rams make great tank mates for discus if you must have other fish in the tank besides discus. They require similar water, foods ect.
Ive also herd of people keeping many dwarfs in close quarters, although this would seriously best be left to some one with significant experience with the particular species. Apistos are found in waters with significant leaf litter and are often packed together very tight in the reivers/streams.-I will find my source.
But yes do quarantine any new arrivals especially the dwarfs because breeders may be less likely to pre treat the fish. I actually had 2 separate dwarfs bring in 2 different illnesses to my tank quate a while ago and I will not add any new dwarfs without a long quarantine. I currently have a pair of apisto. agassizii almost ready to add to my tank when I find a block of time at night(Wednesday).
And yes, that 3 foot tank shouldnt be able to hold that many more fish! 3 feet is not long at all even if it is like 24 inches wide/deep, so I would consider it almost tapped out with a small group of discus and some dwarfs.
What exactly are the demensions of your tank? Its kinda hard to offer advice without knowing really how much room, how many fish and how many gallons you have.