It is very difficult to get seahorses in Canada that are able to exist in the temperatures you are wanting. Currently, there are only H. Kuda, H. Erectus, H. Zosterae and if you are lucky, H. Reidi. All need lower temperatures. This is to help prevent the increase in pathogens in the system. H. Zosterae will only eat live food so are difficult to keep.
Also, it's not just the temperature. The seahorses do not do well in a system tied into any other system because they are not immune to the pathogens that can be present. While they may do fine in the Ocean, in a concentrated environment like an aquarium these pathogens will kill them before you have a chance to get to know them.
Even mixing seahorse breeds is very risky and usually ends with the death of one or both breeds. Seahorses are not cheap and to go to all the trouble of finding some only to have them die is heartbreaking. (believe me, I have lived it)
I would suggest setting up the 15 gallon to house some dwarf fish that you may not be able to see in your larger system. I saw some really cool fish once in Big Al's that only got to be about 3/4" long but for the life of me I can't remember what they were. I remember asking them about these fish and were told they were great for a small tank. I do remember that they were yellow...
