I've actually considered a very similar setup - or rather AM considering a similar setup for our new house. I like the idea of seperate tanks, so I can add on a tank as I see fit. I'm a college student to the budget is low, and I'll be aquiring parts for it until the summer. My plan is to have the return split and have it t-ed off for several tanks but plug the ones I'm not using. That way, if I fill up a 20L or the rubbermaid underbed units that I plan on using, I can just add another one on and put the return line to it. In addition, this allows me to take a tank down if I dont need it. If I have a problem in the tank, I can shut it down, close the ball valve on the return or cap the return off and fix the problem - or save on electricity for lighting it
By the way, I'm pretty impressed with your diagram using powerpoint. It looks like a nice setup.