ArkyLady explained quite nicely at the top how to instantly cycle your QT tank. It's the best way to go, and the easiest.
I wouldn't use feeder goldfish to keep my QT tank cycled. So many of them are diseased and unhealthy, plus goldfish would grow, and eventually outgrow the QT tank. Plus, if you're QTing tropicals, goldfish won't do well in the temp tropicals need. And if the new arrivals are sick, you may kill your goldfish, which isn't very nice.