| If you install only one return upstairs, lets say in the hallway.......each bedroom will have hot air trapped in them, especially if you keep the door shut.
You can either branch return ducts into each bedroom (never return air from bathrooms, laundry rooms, or kitchens) ........or, you could simply put one large return in the hallway and install "back to back" grills above the bedroom doors. Basically, they are just an open hole above the door, disguised with a grill on each side, to allow the return in the hallway to suck the hot air out of each bedroom.
I dont know very much about the various zoning systems, but I expect that all of them will eventually be problematic. Its like anything else, the more complex you make a system, the more there is to break down.
Best of luck to you.
Magicman | 
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