I just set up a QT after getting ich in my display tank. Wow I wish I would have set up a QT before hand. You should introduce any new fish to your QT before adding them to your main tank. Leave them in there and monitor them for about 3 weeks. If they look healthy the entire time you can then add them to your main tank. You should strive on keeping the best water quality in this tank as possible to avoid stressing new fish. I also have a 55 gallon tank, and I opted to get a 20 gallon QT. It was only like 25 dollars for the tank, so there was no real price difference. QT tanks are good ideas because you can watch your fish to see if they have ich, or other problems of that nature. They are much easier to treat when already in the QT, as getting fish out of a display tank with LR is a HUGE pain. I think the bigger the QT tank you can set up the better, as some fish need room to swim.
Good Luck!
-JD