Those are some nice looking tree sponges you've got there in the 'fuge!
Why are you using two return pumps instead of one?
I would recommend plumbing the fuge from the tank overflow (tee off the main overflow drain pipe and use a ball valve to control flow) rather than the return pump(s).
As far as overflow bulkhead size, how much flow are you planning to run through your return pumps to the DT?