The size of the tank depends on the stock you are putting in it. The 50/50 mix is what I do, it works fine. I wouldn't worry about the bioballs, however that might work. I just keep a small sponge in my sump, when I need a qt
tank I take the sponge and put it in the qt
's filter. Don't need to keep the tank up all the time with this method.
However if you are going to be treating with meds, the bacteria will be killed off anyway. Ammonia and nitrite would need to be controlled through water changes.