I'm setting up my seahorse tank (it's cycling right now) and will not be setting up a phosphate reactor... at least not in the beginning. If down the line I keep finding I have issues with phosphate, then I might decide to run one. However, I also plan on having a bunch of macro algae in both the display and inline refugium. I prefer to take a simpler approach when setting up tanks and and try to achieve a more natural balance before I start using chemical media.
When I was doing research about this, I remember a few discussions about seahorse gills being sensitive to GFO in particular. I couldn't find any hard evidence or anything scientific to back it up (as so many discussions around here are based just on just experiences)... If it's something that you could have a concern about, then I would use another phosphate remover that doesn't rely on ferric oxide.
With all that said, I've seen seahorse tanks that use GFO with no reports of ill effects.