[kien]

You can 'T' off your return and send some return water to your refugium and drain the refugium back into the sump.

Matching a return pump to your overflow isn't hard. You can either trial and error it, or just take a poll and ask others what they are using for return pumps with their herbie overflows.

Think of it this way, your wide open overflow is going to flow out whatever your pump is going to put out. So long as your return pump is rated for less than what your emergency pipe is rated for you're okay. If your 1.5" pipe can do 1500 gph, just make sure your return pump (don't forget to account for head loss, bends, etc), isn't pumping out more than that and your emergency will be fine.

I have a mag18 which is rated for 1800gph (probably getting about 1100gph after head loss, bends, etc), and when my 1.5" drain pipe is wide open there is more than enough flow on the emergency (super noisy though). If you're really worried then just use the third drain pipe as a secondary emergency.

You can also put a ball valve on your return line so that you can adjust the flow of the return pump. So if you end up buying a pump that flows more than your overflow can handle, just throttle it back with the ball valve. I wouldn't completely cut the water with that ball valve though, instead, give it an outlet that feeds back into the sump.