If its not already cycled, you will need to seed it with filter material from the main tank (I would also just pull water out of the tank to fill it as well so the parameters aren't different) and as you mentioned make sure its not carbon.
I would probably add water conditioner and allow it to run for a little while before you pop him in there so as not to just stress him out too much.
Keep in mind with antibiotics you will kill some of your biological filter and if you treated your main tank, its possible to have a mini-cycle.
But as mistergreen said, its cheaper to treat 10g over 26g.
Set up the 10g with tank water from the main tank and place some filter material from your main tank in the filter (and some new stuff if you'd like; just no carbon). Get your heater in there. Add water conditioner, let it run for a little while with the medicine in there and then transfer the fish.