I can pretty much confirm what others have said, seahorses should be housed in a species specific environment only. For the most part anyhows, Their are some minor exceptions we've have tremendous success with as other fish with (small gobies, like a clown goby's, a mild tempered watchman). You do not want anything at all to compete for food with them, nor anything that would harrass or have the ability to become agressive. I would put to you that your Angel would not be a good combination with seahorses in any reguards,

They have very minimal lighting needs, just enough to see thier food is sufficient and suport any plant life. As I've heard from DavidP a hundred times the environment for seahorses is very low flow, high nutrient system. If you introduce higher sources of light it will 99% of the time become an algae heaven. Not to mention possible harm to the ponies as the UV will litterally sunburn them.