Hi Paul, nice diagram you've done up. On the surface, looks good but I have a few comments;
Are the assorted dimensions intended for individual units or glass-partitioned 4ft tanks? Is there no provision for pre-drilled tanks with bulkheads or flow-through? In the long run, that can be a life-saver and can open more possibilities for modification as your needs changes.
Try to orientate the pumps' water intake at the top. If not, extend a tubing to where you want the water level to stop. Bottom line is to have the pump still submerged when at the desired level so your pump don't run dry (out of water)
Plumbing - Drain
If the pumps (on the same tier) are not started simultaneously, discharged water will find the path of least resistance. In the case of the middle tier, water from the 1ft tank will surely flow into the 2ft tank before heading to the down tube. I do not see how water flow from one tier is prevented from entering a lower tier (you need more ball valves).
Plumbing - Intake (from holding tank)
Taps/ball valves need to be relocated to the beginning of each horizontal run. Having valves in the same line (vertical up tube) will disrupt water delivery to the next higher tier. (eg. if you shut the valve at the 2nd tier, how does water flow to the top?)
Your setup has a minimum 6ft height and that submersible pump's specification must surpass that overhead. My preference is nothing less than an Eheim.
If using air-driven sponge filters, get a Hi-blow. The model #20 is a nice capable pump for that setup or a #40 if you plan to install more racks but remember to bleed off the excess. Nothing kills that pump faster than constant back pressure.
If you're into the hobby for the long haul, consider stainless steel or chengai wood racks. These are good investment and will probably last longer than your interest
Allow for disassembly of the discharge manifold, using threaded or union connectors, in the event you need to carry out general cleaning, clearing a clog or further modifications.
Based on your diagram, there is no less than 14 pumps, excluding lighting. Plan your wiring, AC points and avoid overload. Water & electricity is a lethal combo.
Just some thoughts off the top of my head. Have fun at the drawing board.
I'm back & keeping 'em fingers wet,