Seahorses are delicate and should be kept in a mostly species tank.
some fish would be way to aggresive towards seahorses and physically hurt them.Others would not give the seahorses a chance to get food because they are much faster than seahorses. They also cant be kept with certain corals because the seahorses would be stung. there are many different reasons for the limited ammount of tankmates.
are you refering to this website? http://www.seahorse.org/library/arti...ankmates.shtml