Your overflow can handle up to 1500gph but I wouldn't want to push it.I would go with something rated 1200-1500gph at zero head height.They will lose around 200-250gph at a head height of six feet.If you split the return lines up,you will even lose more gph so bumping up 1500-1750 would be better.
I do like the say if you are going that high gph,an external pump will run cooler then an in-sump return pump.For external,the Panworld pumps are the best IMO for cost,reliability,noise and heat.
You should always put a ball or gate valve after your return pump. It's very rare that the overflow and pump will be perfectly matched, so odds are you'll have to dial it down. To reduce the amount of back pressure on the pump, you can plumb a T from the pump -- one side of the T goes to the display tank, the other side of the T goes back to the sump. When you adjust the ball valve, it simply redirects some of the water back into the sump instead of just blocking it off.
Let me see if I can explain it better. Like this: Return pump splits off into a T -- one line goes to the sump, the other goes to the display. You put the ball valve on the line that goes back the sump, so when you open and close the ball valve you control how much water gets redirected back to the sump. That's how you control how much water goes back to the display. The more water being redirected back to the sump, the less water ends up going back to the display.
You only need one return pump. I can't think of a good reason to have two.
Of all the pumps ive looked at i like the Pan World. I like the idea of a external pump anyway. I think an internal pump would warm the water up quite abit. In time i may have a extra but to start ill have to just stick with one.