Okay Abe, hows the Tang? As far as a QT tank, 10g will do but as always the bigger the better. I would get a sponge filter and run that in your sump. Then when you plan to buy a fish take the sponge filter out of the sump and place it in your QT with freshly made up SW
. Let that run and when you buy a fish acclimate it to the QT water and QT the fish for a Min of 14 days.
If it's in a rush, take the filter and put it in the QT with water from your DT and add the emergency injured or sick fish out of your tank. This time there would be no need to Acclimate since it's the same water.