Last time I had seahorses was about 8 years ago.
One thing I think you'll find corroborating info on is they like vertical space, low flow and lots of stuff to grab on to like macroalgae for example.
You CAN have other species companions as long as they're not competing or aggressive eaters that will out-compete the slow eating seahorse, It also works out better if you have a larger type of seahorse if going cross-species compaion.
If you're running lots of macro, like it sounds like you will be, water quality shouldn't be much of an issue if biofiltration is adequate, however you can still skim if needed.
I highly recommend as much biofiltration as you can get away with/are comfortable with, (lots of cavernous rockwork works well, as long as theres still open space for the seahorse of course) they tend to leave a lot of food laying around and as much as they need to be fed given their short digestive tract it adds up. (I had a skimmer on my old seahorse setup to help)
Pod cultures or seeding the tank with lots of pods is a great way to keep up with the seahorse's nutritional needs, you can also seed the tank with mysid shrimp for them to graze on.
You can feed brine shrimp to coax a picky or stressed seahorse into eating but you should not rely on it for feeding, due to their short digestive tract combined with the already low nutritional value of BS, they get nothing from it and will in fact die if only fed brine for extended periods. Choose mysis if feeding frozen.
If you notice what can at times look like pealing skin coming off of a seahorse thats otherwise looking fine, don't stress as it's likely just algae growing on them. They move so slow that they can accumulate microalgae at times, sometimes even coralline. (yes really)
I'm sure a lot of that is common knowledge but I thought I'd mention it for the sake of the topic, overall I'd say have fun with it, seahorses are fun even if they are a little more demanding at times than your average fish. I look forward to seeing pictures of the setup. 