I agree with Cuttlekid; seahorses can tolerate a lot more flow (once full-grown) than commonly believed. My H. erectus are in a mixed azoo tank with moderate flow (no powerhead) and are totally fine. To be honest, I'm considering adding a powerhead to my system because they tolerate the flow quite well.
To address Coltrref's point: the gorgonians get used to the seahorses hitching to them. they do not like it at first and quickly retract their polyps, but over time, they keep the polyps around the hitched seahorse extended. That said, one thing to watch out for Cuttlekid, is that the horses hitching to the gorgonians will wear off the fleshy part of the gorgonian and it will die around that portion. you can cut and reattach (I've had to do this a few times) and it will bounce back. One thing you can do is to provide other hitches for the ponies. I have two colt corals that look pretty decent that my erectus love to cling to. The colt corals do not mind it at all and it keeps the horses off of the gorgonians.
BTW, the first erectus is stunning. I really love the cirri. You've inspired me to upload a few pics from my tank today...