I have had pretty good luck with macro's like prolifera, taxifloria, and red grape for hitching posts. The seahorses would hitch to it and hang out. Red grape was kind of the coolest because the one reidi turned from a pale yellow to a nice deep orange.
Other macro's like ochotode, green grape, suction cup and ulva did not work so well.
IMO I would go with live plants as they will also give some aid to filtering water. Seahorse tanks are notorius for nitrate problems for many keepers so having your decor eat nitrates is cool. The taxifloria (while illegal in some states and does have very strict removal procedures) is my favorite becasue the leaves will grow about 18" tall.
I run the flow in my seahorse setups higher then most. IMO low flow is not the way to go. Careful planing on how to manipulate the flow so there are no strong jet stream currents and places to rest is the best option.
I tried keeping my seahorse tanks low flow for years. Always had problems. It wasn't until I got into reef tanks that the importance of flow became apparent to me. I was to trapped in the seahorse world to see out of it.
If your going to go with minimal rock you can get away with less flow. IME it is most important to have flow going through the rocks to eliminate dead spots, so nothing gets trapped and causes other issues.
After going to extremes myself to see just how much flow I
could do, I am now really thinking of going back to a more minimalist approach with just macro algae and a few larger pieces of branch rock. Took me 7 years of keeping seahorses to get here so your way ahead of the game.
Best of Luck Dude. This is a good forum to help you on your journey. Hope you take advantage of it.