Yes, agree with rabbut about the size, its all up to you... 5g,8g, 10g, 20g depends on your fish and availability of space etc.
The method of choice here seems to be to have just this tank (it will be bare-bottom, no substrate) and a heater. Then for filtration you either have some sort of extra filter which you run on your main tank all the time and then set it up with the Q-tank when needed, or you move mature media over to an empty Qtank filter as rabbut described.... basically though its just equipment that you keep somewhere to pull out (although its loads better to have tested it right after you bought it of course!)
Yeah, you go ahead and add ammonia as soon as you find it, hopefully getting the tank to 3 or 4 or 5ppm. Its not gonna cause nitrite(NO2) to drop to zero overnight, quite the opposite, its going to help the A-Bacs produce even more nitrite at first. What you'll be accomplishing however is finishing up your fishless cycle with the colonies becoming much larger and more robust before they drop back to match the size of your first fish population.
~~waterdrop~~