personally i wouldnt even recommend seahorses to anyone other than people who have a lot of experience in keeping fish. They are one of the most time-consuming groups of fish out there. I have kept a couple of species and after a year or so gave them away as they were too time consuming. They are very picky about their diet and the quantity of food that they require, also ensuring they are getting enough vitamins is something for pulling your hair out about too!!!
All species of Hippocampus are now clearly listed on the CITES appendixes so they are being watched as an endangered genus in the wild.
So, if the seahorses are a first stab at fishkeeping I'd forget it and look for something a bit easier to begin with.
As for keeping them together in the same tank, that has been done and if you read back on past posts and do a search on seahorses in this site I'm sure you'll find the information... look out especially for Jimbo (cephJedi) posts
I'm not recomending this but seahorses were my first marine fishes........ I have a small joubini (sp?) with my seahorse. However it tried to kill a pretty large seahorse, it failed but left a few wounds (now healed). Since then there has been no conflict between them except over food.