Rick and I and possibly others have kept seahorses. If possible, get Captive Bred (CB) since they eat well and have fewer health problems. At the least get Tank Raised (TR) that have been trained to eat frozen mysis. You can also purchase the dwarf (H. zosterae) seahorses which get no more than an inch long, but they will need a small tank (2-10G) and a constant source of live food, usually newly hatched baby brine shrimp (called bbs or nauplii on the seahorse forums).
In either case, you will need a mostly dedicated tank. Seahorses have no fear of predators but are easily stressed by fast moving creatures. They will happily sting themselves to death by hitching onto anemones or LPS, will try to play with the crabs and will repeatedly violate the space of territorial fish. They are also commonly burned by hitching to in-tank heaters and sucked into even small water intakes.
All that said, they are fascinating animals to watch, very playful and social. They are my kids' favorite animals and the main interest of visitors to my house. I would highly recommend
http://seahorse.org/ and
http://syngnathids.org for information and helpful community forums.
