I've got to admit I can think of better tankmates for seahorses:grinyes: As much as I loved my box fish (newbie mistake) I would never get another because of the risk that comes with them.
On the other hand, knowing how docile seahorses are, they're not going to aggrevate the cowfish.
Have you considered mandarin dragonets (depending on how long your tank has been set up) or pipe fish?