I have erectus and reidi together in the same tank, but OTOH I know that they were all captive bred and came from the same supplier. My feeling on mixing species is slightly different than Ms. Cran Berry's, but I would caution anyone who is just starting ouut to stick with just one species.
Different horses have different feeding habits, and you have to be prepared for that. For instance, my reidis stay closer to the bottom half of the tank, and spend most of their time hunting pods from the rockwork. When feeding time comes, they prefeer their food to be presented down in their feed bowl, so they can hitch near the bottom and enjoy their meal. The H. erectus I have with them, however, is very active, usually around the top half of the tank and in the flow of the powerhead. She doesn't seem to care for hunting tiny food items in the least; she prefers to be fed her frozen mysis in the water column so she can hunt them down, and she goes absolutely nuts when I give them a treat of live feeder shrimp or live brine. Her method of hunting is definitely focused in the water column, not the seafloor.
So I think as long as you know that the horses came from the same supplier, and you're prepared to take action to make sure everyone gets fed, then yes, I believe you can mix SOME species. You should never ever, however, (ooh! end alliteration!) mix WC seahorses with anything else. Too many chances for parasitic infections to spread, and CB seahorses lack some of the natural defenses WC horses have in that department!