This is turning into a helter skelter with lots of conficting advice. Many are confused.
First you need to understand the measure of moisture in outside air verses inside air. %RH will not work. You must use dew point or grains per lb. of air. Research on the web and give us feed back.
Second. A properly setup a/c with a heavy cooling load will get you to a low indoor %RH/Dew point/grains per lb.. At 75^F 50%RH, you have a 55^F dew point or 66 grains of moisture per lb. of air. The cooling coil must be less than 50^F degrees. Usually this means low speed on the fan.
During low/no cooling loads in any green grass climate, the inside moisture level will be higher than the outside moisture level when the home is occupied. Because the a/c is not removing any moisture and the occupants are adding moitsure. If the outside dew point is +55^F, the a/c is not runnin alot, and you want <50%RH, will need some dehumidification. If any a/c guy can explain how the dew point of the air in the home will be less than the outside dew point without dehumidification, please explain that to me.
Third If all of your inside doors are open, there is no big advantage for individual room returns. If you close a door on a room, a room return will better balance the pressures on that room.
Fourth All homes need a fresh air change in 4-5 hours to purge indoor pollutants and renew oxygen. On person in an 1,800 sqft. home would be about 600 ppm of CO2. In windy weather, most homes get enough fresh air. In calm weather most homes do not get enough fresh air.
A fan introducing 70-80 cfm of fresh air to your home will make a big difference during calm weather. The fresh air should be filtered, blended, and circulated throughout the home. This is a must when occupied.
A whole house ventilating dehumidifier will do all of this even with an over-sized a/c. A properly sized a/c without dehumidifier will not do this. An Ultra-Aire 70H can handle upto 2,500 sqft of home if the a/c is setup correctly and the home fairly air tight.
Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"