WOW, very cool project!
I can't help a lot with most of your questions, but maybe a little:
1. A LOT! I agree in that you won't need as much "turnover" (I hate that term) as we do in smaller tanks, but it would still be wise to keep as much detrius in suspension as possible.
2. Look into marine grade paints that are used for boats.
3. Assuming you can keep nutrients in line I wouldn't worry about doing 100% water changes every week. Honestly once your tank is well established with the refugium and all you won't need to do much water changing at all (look into other means of replenishing necessary nutrients).
4. For this size aquarium you'll want to look into large prop pumps that are commonly used in public aquaria - way out of our league
6. I would go with around 6".
http://www.ronshimek.com/deep_sand_beds.html
7. In
most aquaria (yours isn't most
) mangroves are not nearly as efficient at exporting nutrients as macroalgaes based on the size mangroves we can house:
http://www.advancedaquarist.com/2005/3/lines. You could grow large trees, and it would honestly look amazing if you did. I would utlize mostly macroalgae initially and grow some mangroves (we can only harvest pods here) in your refugium.
8. Uh, a really, really big one! The three names that come to mind off hand for a "commercial sized" skimmer would be RK2 (USA), Sander Aquarientechnik (Germany, similar to RK2), and the "newer" styled MRC (My Reef Creations, USA). I'm sure ATB (Austria) or Royal Exclusiv (Germany) would be willing to build something but that would be big, big bucks. In my honest opinion you could probably get away with just your refugium and water changes, but a skimmer(s) could only help.
I would contact people in the public aquarium industry for more information regarding pumps, filtration, water changes, etc. Joe Yaiullo is the curator of Atlantis Marine World (Long Island Aquarium)
http://www.atlantismarineworld.com/a...ts-people.html that built and maintains a gorgeous 20,000 gallon reef there. He is also here on RC; I would try to get ahold of him somehow.
GL, I'll be following this build for sure
