The first and most important thing to do is be patient. Seahorses are very unique creatures with very specific needs. To answer your first question, the best thing to put in with them is snails, and that's about it. They do best in a species only tank. Another key thing with seahorses is that you need to buy captive bred seahorses, from Seahorse Source for example. Their tank setup is different from a reef setup. It needs to have lower flow, no fast moving or aggressive fish, as I said, preferably no fish other than the seahorses. The temperature needs to run lower, between 68 and 74 degrees. Good species to start with would be H. erectus, H. reidi, or H. kuda. Pick one and go with it. Mixing pipes and seahorses can be dangerous. Seahorses are captive bred and pipes are almost always wild caught, so they have the potential to transfer any number of diseases to seahorses who are susceptible to them. There should be less live rock than a reef. Its a sort of a lagoon/seagrass bed setup. Live macroalgae is great for them. You have to be careful to incorporate no stinging corals, bivalves like clams, anemones, etc.
The most important thing is take it slow and do plenty of research. This is not one of the easier animals to keep, but it is doable if you are careful.