QT tank: All that goes into a QT tank is the water, hitches, HOB filter, and an airline (no airstone, just the big bubbles straight out of the tubing) No LR or sand just incase you have to medicate them. The meds will soak into the sand and LR and it will be no good.
If you want to add corals to the tank, you have to make sure they are not capeable of stinging the seahorses. Most people only keep leathers, zoanthids, and mushrooms with seahorses. Remember, corals need more light then the seahorses and macros do. High light and low flow = bad algaes. MH lights can sunburn seahorses. T5's are good if you want corals in the pony tank. You could add some sunpolyps.... they don't require light and they eat mysis shrimp like the seahorses.
Grape caulerpa is pretty, but it seems to go sexual more then some of the others. Razor caulerpa is a good grower and I have never had it go sexual on me. Another good one is the feather caulerpa. It is very pretty waving in the current. Have you visited seahorse.org yet? They have some great articles and how to section for setting up seahorse tanks. They are a great group of people and they know a lot.
The tank will need to be set up and cycled before you can get seahorses. 6 weeks of doing nothing then another 6 weeks with the horses in QT. Nothing good happens fast in the saltwater hobby. Do you know what kind of seahorses you want? They require different temps. and tank heights. Pick the horse you would like to keep and set the tank up their needs. You can not mix different kinds in the same tank. Very few people have had them live when they mixed them.
Good luck. And Research as much as you can.
Angie