I would agree on the drilling, you can do an over flow but drilling makes for a cleaner and more efficient system. Now your going to drain out of the tank into the sump and then go to 2 sumps?? If I am correct in this........ I would suggest that you combine your sump and refugium and return into one tank. If you have a stand
I would get as big a tank you can fit under your stand and combine your sum,refugium and return all in one. this is a alot better for space and simplifys your system. I hope that made sense. :D