if you are thinking about QT, then yes you want to QT the first fish and all fish. In theory you even want to QT corals/inverts/CUC as well, if you want to be truly careful and preventative.
there are many methods/concepts of QT, choose the one that works best for you. Personally, i like TTM, followed by observation in QT tank for another two weeks with prazipro administered either with TTM or in the QT. but that may no be/work for you. whatever you choose, do it right and follow the guidelines, no use putting in the time and effort to do it halfway.
I also suggest a CUC first, and if you put it in now before the fish..they would effectively be in QT until then. I wouldt add a CUC rated to the size of your tank, id go a bit(30-50%) smaller, but start there. then get a fish and QT, then add to DT. youre on the way to a fun tank!