Advice needed for holes to drill in the 65g

[treesprite]

My Gen X 6000, rated at 1587 gph, is an amphibious pump. I want to drill the 65 in a way that I can feed the pump in EITHER a closed loop fashion to use externally (greater flow rate, less noise, less heat, less microbubbles), OR internally like I am now, to accomodate any future possibility of changes in circumstances.

 

My actual filtration return (not intended as a tank flow source), is a 258 gph rated pump in the refugium (drilled sump overflows to fuge). If I use the Gen X externally, I'll put a small return (200-300 gph) in the sump for insurance and maintanence purposes.

 

I need to know how many holes of what size to drill to accomodate those 2 small pumps, in addition to the hole for the Gen X. I am assuming that the Gen X hole size will be fine whether it is connected or goes to the sump with the Gen X in the sump. However, if the Gen X is used in the sump, there will be no second small return used. I'm assuming here that regardless of how many of what size holes, the amount of water draining to the sump will not be greater than what the pumps are returning to the tank, but is that a correct assumption, or do I really need to keep using the small extra pump if I end up using the Gen X internally after drilling?