Interpolating from the chart on that web site I come up with about 1320gph at 9' of head height. With the bends and the T you are going to have you should be right on the mark for a return pump.
With a 9' head you should use mostly pvc
up to the middle of the tank and T off from there to each return.
You can not rely on a return pump to provide all the water flow necessary in the tank. I would still put a couple of powe heads in the tank, or build a closed loop system.