Somehow I missed this thread. I have a few cautions about the set up that you are planning. The biggest is temperature...seahorses do best in water temperatures under 75°F. Some folks have kept seahorses in temperatures that are higher but very few have done it successfully long term. With your smaller seahorse tank being plumbed into the larger reef tank, the seahorse tank will undoubtedly be 78° or higher. The only way to keep seahorses in those temperatures long term is to only feed live food, gut loaded mysis, or ghost shrimp and gut loaded live brine shrimp.
My other caution is in regards to your seahorses sharing water with the fish in your reef. Seahorses do not have the same immune protection as fish. Some of your fish are probably wild caught and most of your fish will be from different sources than the seahorses. The ponies will be exposed to many pathogens that they have never been exposed to and because of their deficiency in that area they will be at risk of succumbing to them.
My final caution is in regards to flow. Seahorses actually do best in tanks with high, moderate and low flow areas. They don't have the stamina of fish but they actually are better swimmers than most folks know.
If I were you, I would set the seahorse tank up as a stand alone system. I did what you are planning and eventually I separated the systems and ran the seahorse tank alone.
