I have two ac/heat pump units supplying my house, and judging from the speed that the house gets heated/cooled, I would say that the units are oversized for my house. That being a completely unprofessional and uneducated opinion of course.
The units are electric. Would adding maybe two registers down in the basement seriously degrade the first floor's heating/cooling? One unit is in the basement feeding the first floor, the other in the attic feeding the second floor.
Also, would I require a return air duct? The area downstairs includes a play area, so although we would not "live" down there, it would be occupied for a few hours a day.