I run a Speedwave 1320G - 24 volt DC speed controllable return pump and as mentioned dial it back to what you require or run it full out with a couple of T's after the pump to run reactors or what ever and or just dump extra flow back into the sump. This pump has worked great for me on my 90G reef with a 50G sump and was amazed with how quiet it runs compared to the Magdrive900 I had before. I would go with 1" for both your drains, herbie and your emergency overflow.
