Overflow bulkhead size suggestion

So I’m looking at ordering my bulkheads for my 150 gal build. I know I want to run a bean animal overflow, thinking one overflow box on the left side of the tank and return on the right.
Questions
#1 What size pvc pipe to feed the sump with? I see brs has a ton of different sizes so unsure what size is proper for this use. Would I rather go slightly larger then needed and use a gate valve to adjust the flow?
#2 how close is to close to drill the holes? I don’t want the glass to have a weak point because the holes are to close
#3 I’m assuming I will want the bulkheads as close to the bottom of the overflow box as I can get, then an elbow coming up, then my primary secondary and emergency lines plumbed into the elbows that’s inside the overflow box that’s inside the tank?
#4 slip or threaded bulkheads, bts has them in every combination you can think of, I’m leaning towards slip on inside the overflow box and threaded on the back side of the tank that way if the bulkhead ever has to be removed I can easier
I’m sure I’ll have all kinds of questions about the plumbing on the outside of the tank but figured I would start with the plumbing inside the overflow box and go from there
Thanks in advance
Matt