Well my opinion is only that, my opinion, and others often have differing viewpoints. I'd suggest doing a LOT of research before you go any further with this hobby as it is NOT just like keeping salt water fish in a reef tank.
First, I'd recommend setting up a separate system, one not tied to a tank with other fish in it, as often seahorses will succumb to pathogens introduced by other fish, pathogens the seahorses haven't grown up with.
The number one cause IMO, of seahorse losses is bacterial diseases due to water quality issues that present themselves in seahorse tanks due to their unusual eating habits. The bacterial problems are most often exacerbated by temperatures above 74°F as higher temperatures really escalate the growth of bacteria exponentially. I don't know SPECIES that do well at higher temperatures in tanks, only in the wild where the water is constantly changing, not captive as in our systems. There can be individuals within any given species that might fare better than others at warmer temperatures, just like we humans have a range of capability of the body to weather the pathogens we get exposed to.
Tankmates need to re researched as well as many fish can affect the seahorses negatively. Also, some corals can be damaging to seahorses, and, some sps corals can fail due to the water quality produced by seahorses. Other corals can be damaged by seahorses hitching to them. Clams are not good tankmates as many other reef tank choices are often not.
Water flow should be in excess of 10X, taking into consideration that no flow area should be able to direct the seahorses against anything to damage them, and, have hitching in all flow area types so they can choose where they want to be at any time. The water flow needs to be sufficient so as to keep uneaten food/detritus in suspension long enough to be captured by the mechanical filtration so that it can be VERY FREQUENTLY REMOVED so as not to provide food/bedding for nasty bacteria.
IMO, for best chances of success, water quality needs to be kept better than for any reef tank.
I've not treated seahorses in the quarantinel tank, preferring to buy from a source that breeds them properly so that the quarantine is just a waiting period to be sure that they survive the trip to me OK.
I personally don't feel that one species are any easier to keep than another, unless however you wish to breed them and at that point I have to say that benthic producing fry from seahorses like H. erectus are easier than pelagic from H. reidi for example.
There are lots of things to learn but I've covered the basics from my point of view.
I'd recomment reading Pledosophy's post number five in this thread:
http://reefcentral.com/forums/showthread.php?t=2274878
I have a page on my site about my thoughts on seahorse keeping and it is at:
www.angelfire.com/ab/rayjay/seahorsekeeping.html There are links at the bottom of that page that lead to more information.
I'd also recommend paying extra close attention to any posts you see written by Dan Underwood of seahorsesource.com, the best and most helpful breeder on the seahorse forums.