The reason they dont do well with other fish is because their needs differ tremendously. They are very slow, methodical eaters. Will literally stare at a piece of mysis shrimp for 5 minutes before deeming it worthy to snick up. Any fish that eats at a normal rate would quickly get all the food before the horses could, and they would starve. The other thing is low current, no bubbles in the water ...not your average thing for a saltwater tank.
Seahorses do well in eclipse systems. You might want to consider that at some time in the future.