Tropical seahorses have been kept at temperatures higher than 74 degrees, however higher temps come with a higher risk of disease and death since the bacteria reproduce quicker and mutate to become more aggressive at temps higher than 74 degrees. So, its just a calculated risk that you need to decide if you are willing to take.
You will have to feed the seahorses, the pod population in the fuge will not be enough to feed them. The good news is that if you buy aquacultured seahorses from a reputable facility like seahorse source or draco marine, the seahorses will already be trained to frozen mysis, so feeding is much easier. Although you still have to feed 2-3 times a day.
Be aware that your fuge will no longer be opperating as a fuge once you add the seahorses. It won't work as a refuge for breeding pods and other small critters, since the seahorses will decimate the population quickly, even if you are feeding them regularly. And, it won't really work for nutrient export, since the seahorses are nitrate factories, so they cancel out all of the nitrate processing of the macroalgae. So, if you need a fuge on this tank, don't add seahorses. Thats why most people just use a seperate system for seahorses. With the refugium not functioning as a refugium anymore, and instead just being an attached seahorse tank that adds more nitrates and waste to the reef, it makes more sense to make it a seperate system, where the reef is free of the waste produced by the seahorses, and the seahorses can be kept at a better temperature.
If I were you, I would look into adding a reef-type pipefish like bluestripes or dragonface pipes instead.
Also, will people even be able to see the fuge under the tank to admire the seahorses?
