if you havent bought the tank yet i would just look into a prefabricated split tank. theres a bunch at my lfs that house say a planted tank on one side and a cichlid tank on the other, i havent checked the maker of the tanks but i did notice a seachem sticker on the tank, not sure if it was just an ad or if seachem makes these tanks but id look into that first.
if i had already bought a tank i might look into making acrylic baffles to section off tank? id definitely put slits in the baffles to allow for (some) of the tank flow to direct its way to seahorses, unless your going to put a small powerhead on their side too. but baffles in a main display scare me, id be afraid of it putting pressure on the glass.
You don't want to allow the water from the reef side into the seahorse side. They need sterile condition and any hitchhikers to would do harm. They're also very susceptible to bacterial infections and need to be kept at lower temps than your reef tank. I'd suggest just setting up a seperate tank for seahorses...
I kept H.Reidi in my soft coral reef without problems and have found the ones from ORA able live find at reef temps. They are collected from different areas and come from temperature ranges of 75 to low 80's. . Now after saying all that cooler temps are probably better because of issues bacterial infections are less likely at cooler temps. I never had any issues with it maybe because all my seahorses came from ORA.
The thing about splitting the tank is you will have an eyesore on the front panel I think you will get tired of over time seeing the divider. A dedicated tank is best and can be done at a decent cost.