fish question

I think you need to lookup compatible fish for your current stock and go from there. If it isnt over stocked already.. I'm not going to be the judge of that though. hopefully someone more experienced will shed some light on this topic for you.

seahorses probably wont do much swimming, just hovering around different spots. and between them and the mandarin dragonettes your gunna be buying or breeding a massive mount of live coepods. not many reef fish are surface dwellers, since all the action is down on the reef, but im sure someone will chime in with personal experience

Hopefully you have not already purchased your stock suggestions. If you are going to stay with wanting seahorses, your tank is good for a pair of Hippocampus erectus or kuda. You do not want to add other fish unless it is a pipefish. Seahorses are slow eaters and the reason you don't want other fish with them is that they will compete for the food. To keep seahorses you also need to be ready for any sort of seahorse illness and if you go to syganthid.org or seahorse.com you will get suggestions on what meds to have on hand. Reason being that when a seahorse gets sick you can't really afford the time to go out and find the proper meds for them. Just MO, hope the info is useful. and I have kept seahorses so this is from my experience and research.

Be careful.. while a mandarin may seem slow to you now, when compared to a seahorse they are lightning fast and will easily out compete seahorses for copepods. I would be very worried about putting a mandarin with seahorses unless you have a very large refugium for copepod reproduction.