First, I consider there to be a considerable difference between a QT for new fish acquisitions and a QT for treating specific disease on a fish or fishes. As you mention new fish, I will confine my comments to a tank to quarantine new fish acquisitions.
If you have the space for a permanently-running small aquarium, this is absolutely ideal for this QT. A standard 5g or 10g should be sufficient size; as the main tank is a 20g, you will never have too many newly acquired fish for a 5g. This tank needs a heater and a filter. A single sponge filter is adequate; I use these on my 10g, 20g and 29g tanks normally, so running a separate air supply to the QT is easy.
Floating plants should always be included as they shade the tank (new fish can be light skittish) and will easily assimilate any ammonia/ammonium; this will require an overhead light, but it need not be fancy. I have a sand substrate, just half an inch or so; bare bottom tanks are not a good idea, as there is likely going to be a bacterial problem without a substrate to serve as the bed for the necessary bacteria (and we are talking several species of bacteria, not just the nitrifying). A chunk or two of wood, rock or other decor will calm fish faster than empty space. I have a handful of common snails, bladder, pond or Malaysian Livebearing; you may have to drop in some food for these, as without fish this will be in very short supply.
Introducing new fish to this tank is like introducing them to their established display tank home, and they will without question settle down much faster than they would in a sparse QT. That means less stress, and less chance of ich breaking out. You may have to use a comprehensive plant fertilizer now and then, as without fish the natural plant nutrients will be very low. Obviously this tank will never need to be "cycled" with the floating plants.
