Although many think Qt is all about the new fish bringing some pathogen in, and it is, but it is also about protecting the new fish from something your current fish may have built up, or have partial immunity to.
I usually add water from the tank the new fish is going to be placed into without meds, gradually to the QT tank and observe.
Although your current fish may have partial immunity, if the new fish gets infected full blown while in QT, the pathogen could overwhelm the current fish immunity in a closed system.
Better only a few, than the whole lot.
A friend of mine who runs a fish exhibit at a zoo, QTs all new fish, for 6 months.