Glad you got through the leaking tank and found a good solution. I know from experience that this is a very stressful situation.
I was looking into stacking tanks myself, and did quite a bit of research about tanks draining into each other. One issue I found with this is that it is difficult to isolate/separate a tank, if needed. If your fuge tank cracks or busts a seal the whole system is at risk. It may be better to have the display and fuge drain into the same vertical pipe that goes into the sump. This way you can install a bypass, meaning that you can turn a valve and have the display drain directly into the sump, bypassing the fuge. Hopefully this makes sense..as I said I was designing a system similar to what you are doing. You can check out mode aquariums website, I believe they sell a commercial stacked system that works somewhat like this. The return is the same idea.
A space saving solution for an ato reservoir I have thought of is putting the reservoir directly inside the sump. For example- if you had a 75 gallon tank as a sump, you could drop a 10 gallon aquarium right inside of the sump and have water flow around it. This all depends if you have baffles etc, but you get the idea ..and could get creative depending on the layout of your sump.