Its been done. People actually use toadstool leathers as feeding dishes for the seahorses.
Do a search on seahorse.org you'll find plenty. Also there is a video of a seahorse in a ricordea/zoa tank someone has. The only things you need to worry about for tank mates is amount of flow, competition for food, and the toxicity of the stingers in coral.
The only seahorses not kept with softies/coral are dwarf seahorses because they end up getting stung with certain corals or eaten because they are so small.