[lcstorc]

Unless you are going with dwarf ponies I would go no smaller than a 24 and if you go that small no more than 4 ponies. Two would be better.
They like low flow and lots of things to hitch on. You can put live plants in the aquarium for them to hitch on or long limbed corals like gorgonians. That is most of what I have in the pony tank.
You want the normal level of LR to preform the denitrification and a skimmer would be great.
Be sure you get captive bred seahorses as they will already eat frozen mysis. Wild ones will only eat live food at the beginning and you have to slowly try to convert them and it may or may not work.
Also they need to eat several times a day and they eat very slowly. You can try a feeding station but I never got that to work. I would put a couple of mysis in and whatch them eat them and continue until their eating slows down. Generally people recommend feeding 3 times a day but I did twice and mine were fine with that.
Because of the slow way they eat you end up with a lot of wasted food and you need to adjust your maintenance appropriately or your levels go off quickly. You can keep snails and shrimp fine but I wouldn't keep crabs.
You could keep any coral that doesn't sting and I would definitely not put an anmeone in the tank.
Finally don't underestimate the need to keep the tank temp down. The higher temps allow bacteria to grow and infect the seahorses. I believe this is the cause of my ponies demise. I won't be getting them again unless I can find a way to keep the tank between 72 and 76.
That is all I can think of at the moment.
Also you probably want to check out seahorse.org. It is a site specifically for seahorses and there are a number of FAQ sheets and a forum all about seahorses.