In my opinion, the only way to keep seahorses is in a tank by themselves. As others have said, they are so incredibly slow-moving that even the slowest-moving fish will eat all the food before they can get to it.
I don't think you can really even utilize powerheads with seahorses. If they don't get a chance to wrap their tails around something, the current would blow them all over the tank.
This last part is totally opinionated, but I really do not see the point of keeping seahorses. They are totally inactive, sometimes sitting motionless for hours and hours.