This is copied from a very popular, dedicated seahorse site. It is in their "Getting Started" file:
"Tank Mates:
These include but not limited to: blennies, gobies,draganets, mandarins, percula clowns, some butterflys, tangs and wrasses, soft and hard corals, plants. Know any others ? We'd love to hear from you."
I have kept seahorses with a pretty wide variety of tankmates (and have gotten some guff for it). The only problem I have had is with a yellow damsel, who picked on all fish.