Not a sea horse keeper or expert, but I would believe not in the fish department. I think the sea horses would either be starved in trying to get food or be bullied. Crabs and shrimp might be be ok, depending on the kind you choose. Again, not sure. Here a bump to see if anyone can have better advise than me.
Actually, crabs, shrimp, clams, corals that could eat the sh are a no go. Fish can be kept with them, but you have to be very careful with the selection and feeding. Right now, my sh is with Mandarins, a Dragon Face pipe and two Bangaii Cardinals. I take a lot of time to make sure the sh gets fed well.