I had a skimmer about that size and, using my DIY
venturi, I got minimal results with a Mag 12. I'd say Steve is on target with the Mag 18 although some experimentation will be needed. If you oversize the pump, you can always put in a valve to cut the flow if needed. You might try the Rio as a return pump, but I don't think you'll be happy with the results. According to the info I have, the Rio 1400 flows 402 gph
@ 0' head and the shut off is 6' of head. The best way to figure head pressure is to go to the pump manufacturers website and find the flow charts. Add up the height of the return...then add 1' head for every elbow in the line. Compare that to the flow chart and you'll be very close. The equation for figuring head is somewhat complicated and, although I can eventually muddle my way through it, there's no way I could explain it to someone else.