Tank raised seahorses are a lot easier to keep from their wild counterpart. If you can wean wild seahorse from live to frozen, that's good as well. It's also dependent on the type of seahorse you're looking to get. Seahorses will prefer a tank that is taller with lots of vertical space, so get a tank that is tall. They don't like turbulence in the water and bubbles are a no no.
Tank mates for seahorses are very limited in choices. Generally, they're kept with other seahorses and pipefishes. You also need to plan for things where they can latch on to. If you train them well enough and they're weaned onto frozen food, you can use a feeding tray and just let them dine in it. You should always enrich you frozen food supply. If they're picky, you can try partition an area of the tank with a divider. Throw live rock and chaeto algae in there and grow your pods there. The pods can swim out and you can watch the seahorse hunt. It's quite fun. A refugium is another choice but the partitioned area is better if you want to see some action...If you run short of pods, you can try live food (mysis, gut loaded brine, gut loaded shrimps, etc.)
You can't feed your seahorses only once a day. They need to be fed several times a day because they don't have a stomach. This is another reason to wean them onto frozen and leave it in a tray so you can clean it easier.
As for starter fishes, it's hard to choose if you want to transition from the fish tank to the seahorse tank. That is, unless you get rid of the fishes that aren't compatible down the road. There just isn't that many choices for seahorses. So, my question is do you want to transition or are you just looking to get some experience and then donate the fishes later on and focus on seahorses?
Think with logic and rationality more than emotion. Act with moderation and consideration. Contemplate ideals and realistic goals and weigh out possibilities and options. Temper not with personal delusions or false hope but learn to accept and move on.