It's not necessary, but if you don't quarantine new fish, obviously you run the risk of infecting all your fish to an incoming disease.
However, it sounds like you've just done a fishless cycle and are now ready to add fish. If that's the case, then there's really nothing to protect, so quarantine isn't needed.
And, for info, the minimum size of a QT tank would depend on the size and number of fish you put in it. 10 gallons is a popular size (especially if disease does show up, as most medicines are dosed for 10 gallons of water).