You want the water to be no higher than 74 degrees to be exact. In a small tank, you'll probably need a chiller.
True Captive Bred seahorses (Like from seahorsesource.com) will eat frozen mysis and will need to be fed at least once or twice a day. But dwarf seahorses, even CB ones, will only eat live foods, which will require you to hatch large amounts of brine shrimp every day.
Seahorses are actually very social creatures, so it's good to keep them in pairs. But they are so peaceful, most other fish pick on them and injure them. They can be kept with small gobies and some other very peaceful fish.
Seahorses need good water circulation to keep their tank healthy. What they don't like is a strong jet of water. You can create a lot of circulation in your tank with spray bars and gentle powerheads without blowing the seahorses all over the place. I have over a 30 times turnover rate in my seahorse tank, and they are fine because the current is gentle.
Seahorses hitch on just about anything. I bought a few fancy plastic gorgonians and plants, and I even have some live gorgonians for them. But they spend most of their time on my airlines, tubes, and cords, silly things.
Seahorses are just like any other fish when it comes to commotion. As long as no one is pounding on their tank, they should be fine. Mine are quite friendly and investigate anyone who comes to look at them.
A 14 gallon tank is not big enough for any seahorse except dwarfs. Unfortunately, these are the ones that require live brine shrimp 2x daily. A 30 gallon tank is the bare minimum for most species, but you could "get away with" a 20 gallon for H. fuscus.
Most corals are dangerous to seahorses. Stay away from anything that has stinging tentacles or could eat them, like brain corals. Most soft corals, Zoas, and gorgonians are safe for seahorses. I don't think any corals are safe for dwarfs, though, I don't have dwarfs, so let a dwarf expert chime in there.
If you really, really want seahorses, I'd recommend doing the research and getting the right tank and equipment for them. It took me a couple years before I was ready for them, but I'm glad I waited and did it the right way.
As for your empty tank, there are lots of cool things you could do with it. Personally, if I had an extra nano tank sitting around, I would make a cool shrimp/crab tank with some corals. There are a lot of new and interesting crustaceans in the hobby right now.
