My thoughts are that if you are setting this combo up as a true refugium, 1200gph is WAY too much flow and will end up disturbing the plants and allow little "contact" time for nutrient removal. I would aim more for 500 - 600 gph for a fuge setup. A sump setup, however, would be fine at 1200gph.
You'll find that your overflow will perform up to the output of whatever pump you are using. If you can only pump so much back to the main tank, then you can only have that much water overflowing, because if the overflow was to go faster than the pump. your water level in the main tank would quickly drop below the teeth on the prefilter box and no water would get siphoned out.... and this can't and won't happen. You can have any pump on there you like and the overflow WILL match it (up to the overflows rating of course).
It said it came with a 630gph pump. That will be perfect for the fuge. I wouldn't change it if you are set on having a fuge. It is overkill on the overflow however, but it should be fairly quiet having both those drains and splitting 500 - 600gph flow.