10-15 gallons works okay for most fish. I use a small filter, small powerhead and some PVC
for hiding. I have never needed a heater, but this year have decided to add one as we are allowing the house to cool down more during the winter.
Your QT will not be cycled at first, so you have to watch parameters and be ready for water changes. Once you have your display going, a lot of people will keep a filter pad in there to collect beneficial bacteria and then add that to the QT to help biological filtration. Depends on whether you keep the QT up and running all the time or just set it up for new additions or as a hospital tank when fish are sick. It is money well spent though.