Awesome. I got a top for this unit with lights, just haven't added it yet.Haha I got 2 units like that one but mine are enclosed with front lids and lights in all 3 levels.
I'll be using sponge filters for all the tanks besides the display unit, it's has it's own filter. I'm actually making my own filters and when I complete the design for them I'll post it, to get some opinions on them. I'm modeling them from like those corner filters you find. Using lava rocks and/or bio rings and floss inside the filters. All I have to do it build the shell of the filter.If you are going to be using sponge filters (or other air operated filtration) and you have the available footprint you can stagger the tanks to fit more in. Then you don't have to leave the gap between each level. You can add at least one whole extra row to the rack then. Access is a lot easier.
Of course this doesn't work if you are going to use HOB filtration as you need to leave access to the filter.
6" is pretty tight.
You could also use 1" square steel tubing to run your horizontals to free up more space. It won't bend under weight. Just dado cut the 2x4 uprights to fit the tubing in a notch and then drill a hole through it and secure with a bolt.
You will want to paint the steel tubing and seal it to prevent rust.
Andy