Well, I too had the same question and did some searching. Males may be aggressive towards other males of the same species. If the tank is large enough then there may not be as much problem.
They are slow fedders so that is the big problem. Some good additions may be a Pipe Fish as well as a Mandarin Goby or any slow eaters. Be careful with crabs.
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Seahorses usually do best by themselves, but may be ok with similar fish like pipes and files. I don't see why different species would cause a problem providing they have enough room.