What size tank and is this RR (built in internal over flow) or External Overflow Box?
I am assuming tank has 1" bulkhead to drain water to sump. Hooked up Mag 7 (700 GPH) to such system w/o problem but NOT MORE. If memory serves, 1" bulkhead can drain about 600 GPH w/o any restrictions. Since Pump will loose some capacity due to head pressure, never had tank overflowing. As you said could add valve to control but rather recommend to split the return using 2 valves so some water can just return back to sump or to other equipments (UV, Refuge, etc). Closing valve on return piping may put stress on the pump.
If have long tank, can even split three way so you have 2 returns w/ independent valve (one on each corner thus better distribution thru out the tank if feasable) and one valve as control(bypass).
I always prefer to go max as you can (price difference b/n mag 5 and mag 7 is minimal).
Some may used One bigger pump to accommodate Protein
skimmer and UV. Personally always used independent pump for Protein Skimmer. It is easier to maintain this way w/o disturbing return system IMO.
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