I kept seahorses a few years ago, and with lots of knowledge and preparation, it's definiately doable. Since it's been awhile, I'm a little rusty on the specifics, but I can share what I know
One of the keys to keeping seahorses is buying captive BRED seahorses. They will be eating frozen mysis, and they are less prone to having parasites and bacterial infections. I would also familiarize yourself with the "seahorse medicine cabinet". These are supplies you will want on hand for specific bacterial and parasitic infections, pouch issues with the males, and tube feeding. The thing with seahorses is, if they go off eating for more than a day or two, they have to be tube fed. The best advice I can give is just do TONS of research, and be prepared.
I had a 29 gallon tank with live rock, heater, a HOT Magnum filter, and did regular water changes. There are lots of different set-ups you can have, though. I had two pairs of reidi (these, along with erectus and kuda are the more common tropical species available). I believe I had a clown goby in with them, and maybe one other peaceful fish. If I remember correctly, you'd generally only want to put in pipefish with captive bred seahorses if the pipefish were also captive bred. They can be affected by the same pathogens, and I think some people don't want to risk exposing the seahorses.
There is alot to learn about caring for seahorses, but it's worth it.