Go to syngnathid.org or seahorse.org, visit their library and read. See also the sticky at the top of this forum.
I would go with a minimum of a 30g tank. My 3 small seahorses (4.5") really like my 40. A larger tank gives you more control over water quality.
I can't tell you the good and the bad of each species as I have only kept one. Tropical horses like H. kuda are easier because the tolerate warmer water temps than some of the other species.
Go slow and make sure you have a mature system before putting horses in. If you are seeding your new tank with rocks, sand and water from other systems, you can spead up the cycling process a little.
I waited 3 months before putting horses in my system.
Go forth and read, and then come back with more questions.
Fred