You're right about to corals/livestock, we could definately use them in the main tank. My comments were specifically about the seahorse tank. I believe a 75G is 48x18x20. 4 feet in my opinion is too long. I think we need to utilize the space that we currently have, and I would not want to be doing any demolition/construction of walls in the proximity of the main tank.
From my understanding of our original discussions of the sea horse tank, and the advice from people who have kept seahorses, it would be best to keep the seahorse tank simple. Just the tank, and a canister filter to run carbon and provide the low flow that is necessary. Overflows, sumps, powerheads and skimmers are pretty unnecessary, but the real issue is that they add microbubbles which can be very bad for seahorses, and we dont need much flow.
The lighting would be fine, although maybe overkill, as we orignally talked of keeping only some ricordia and such in the tank. If we are going to take a different direction, with large seahorses and baby leafy seadragons, then maybe that setup would be fine. I guess we need to start there, as in what is the main goal.