You might check out Eric Borneman's book; there's a section of a chapter on water flow. You'll get lots of good info there. As far as your updated plans, I'd go with a much bigger pan world for a return pump. The quiet one pumps are okay, maybe among the better submersibles, but they run hot, as do the mag pumps. Stick with pan world; use the non-pressure rated ones. Maybe use one that has about 700 pgh for your return, then another one size up for your closed loop with multiple outlets. You'll get good, consistent flow with that. One problem when figuring flow is that there's usually much more head pressure than you think due to elbows, etc.. and especially junk in the lines after a while.

There's nothing wrong with southdown. Many thousands of award-winning reef tanks have it. You can put some reef rubble (like garf grunge) on top for a while until the sand is settled and seeded, at which time it's less likley to get blown around.