I'm JennM and I'm a seahorse keeper
It can be done in a 10 g, but do you want to?
Unless you're keeping dwarfs (which would be lost in a 10), unless you're getting a 10 tall, it won't be tall enough for horses to mate. Although I've read that they CAN mate (there's a will there's a way) in a 10, horses prefer lots of up and down room, versus side to side room.
Your filtration sounds fine, just make sure there aren't terribly strong currents. I've got lots of flow in my tank but must is up near the top so the horses don't get blown away. Water circulates up from the bottom, but there's no direct current down there. I've got all my paraphanalia in the sump, but on my first horse tank, a 25 Eclipse, I had no problems with intakes or heaters. The OR horses I had at the time seemed to prefer the intake tubes and power cords to hitch on, instead of all the sponges and caulerpa I took great pains to provide for them. My WC horses have never given the "equipment" a second look, they love the rocks, caulerpa, sponges and each other to hitch on.
My horse tank is a 37...24 x 24 x 18 and I've got 2 mated Brazillians(Hippocampus reidi) and one female H. kuda, and a juvenile reidi or kuda (can't quite tell yet), as well as a Gulf Pipefish (Syngnathus Scovelli).
My male horse gave birth on Saturday, and got pregnant again on Sunday.
By "medium sized" horses, I'm guessing you mean something like H. erectus or maybe Ocean Riders. They might "fit" the 10 at first, but they will get taller, and will need more headroom than a 10 long will provide, IMO.
Are you planning on wild caught or tank raised? Tank raised are expensive, but will eat prepared foods. Wild caught need a variety of live foods, and can be trained onto prepared foods, but IMO they do best on a combination of live and prepared. It's a challenge to aquire healthy WC horses, between crappy collection procedures in many places, and wholesalers and LFS that don't feed them properly, many are in sorry shape by the time the hobbyist gets them.
It's very rewarding to keep seahorses, and a challenge to try to rear the young if and when they breed, but PLEASE...do LOTS AND LOTS of reading and research before you take them on. They are definitely "doable", but they are higher maintenance than many other aquarium fishes.
HTH
Jenn