Zoas would be a nice addition for a seahorse tank. They are peaceful coral, and don't release any sweepers or anything. They have toxins, but i'm not even going to get in to that because you don't need to worry about it. Just don't drink the water, and always wash your hands after handling them. But that goes without saying for just about everything.
Anyways, salinity is typical, 1.023-1.026. Some like high light, others like low light, it really all depends on the morph. They generally do well under all type of lighting, PC
, T5, LEDs, etc..
You do not need to feed them at all, as long as you have proper lighting.
I'm not sure what you mean by how big/small. They come in all different sizes, some small, and some have large polyps. It's up to you which one you get. Whatever strikes your eye as appealing I guess.