[SeaHorse_Fanatic]

If you have the water from the display come down into different skimmer box chambers, then you can drop two or three skimmers into their own section and not worry about one interfering with the other. With 12' to work with, I would even go with four skimmers each rated for say 400+g so you have enough capacity. You could add splitters and ball valves so that the flow can be diverted to a different skimmer box chamber if you have to take a skimmer out to service so the water is still being skimmed. This would also allow you to add more skimmers later and then open up a valve to feed it.

With 3' of width to play with in the sump, then I would create a channel in the back to allow the skimmed water to flow down to where your return pump sits while the front 2' serve as your skimmer box. So if I was build a custom sump, my skimmer boxes (where each skimmer sits in water with a constant level) would be say 2' x 2' and the side panels would be a few inches taller while the back panel would be say 10" (or whatever is the recommended water level for the skimmers chosen). Then each box has its own feed line with valve and all the cleaned water would go out the back and flow to your return.