Hello Jen, and welcome to the obsession.
I'm VERY glad to see you wanting to research before buying seahorses, as most seahorses die because people buy before finding out the needs that will provide the BEST chance of success in seahorse keeping.
Some basics are: Minimum 29/30g for one pair of standard sized seahorses, with an additional 15g for each additional pair. Keep temperature between 68° and 74°F to lessen the chances bacterial infestations/losses which seahorses are so prone to, unlike most salt water fish.
While sometimes it works out mixing other fish with them, there is a limited list of those suitable, and, you still stand a chance of passing pathogens to the seahorses that they haven't grown up with, often leading to deaths.
If you want a tank with corals, again, you are limited to non stinging types that can handle "not so clean" water, and the lower temperatures.
There is a lot more information you should know first, but it can't be handled all in one thread without the thread getting too big to be useful.
I have some links at the bottom of "My Thoughts on Seahorse Keeping" linked below in my signature, that lead you to some basic knowledge, (and explanations for lower temps) for seahorse keeping.
You may have to register at seahorse.org to view some of them but that is a GOOD thing to have to do if you want to keep seahorses.
