dont put them with stinging corals, or any aggressive fish. ideally you only want to put them with pipefish, mandarins, clown gobies, and other peaceful keep-to-themselves fish. i have a pair of h. reidis in with my clowns, and a few other fish, and i have not had any problems, but the horses were in there first and they do not having feeding problems because they feed from my hand and/or have a clam shell i put food in for them.
as for putting the horses in a 120, i think that owuld be awesome. there are plenty of corals, inverts, and such that you can put with the horses, but just keep in mind that they are very slow eaters and must either be fed separatly or fed in a feeding tray like a shell or leather coral with an indentation.
if you plan on just using it for seahorses, then i am very impressed and would love to see pictures of the tank when you are done. good luc and hope i helped a little