wouldnt be hooking up to your central a/c unit while it might work ok if your exhuasting the air to another room it wont work very good exhuasting the heat from the lights outside the house as you will be pulling enough air from the house your central a/c unit would be running all the time trying to cool your house down. ideally you would ventilate the lights with an intake and exhuast that gets air from outside the house as its not dumping hot air inside the house for your central a/c unit to cool.
and your 12000 btu a/c unit would cool the grow room while the lights was on with no room ventilation, and you could ventilate the room with the lights off when your not using c02 and there is no use of running the a/c when ventilating the room.
still dont think the 495 cfm fan will be large enough even more so with 3 lights hooked inline although it requires more tubing each light having its own intake would work better.