One inch of Freshwater fish per gallon of water is accurate but goldfish have a much higher metabolism. For all goldfish the rule is 7 gallons per inch of fish. Yes, even more than salt. And I certainly don't want to offend anyone here BUT, using fish to cycle a tank is murder, even a "feeder" gold fish. A life is a life (no, I don't go to church).
It's more humane to just pee in the tank once a day for a week (and no I am not kidding), the ammonia in your urine will cycle your tank.
I feel people in stores should be taught correctly when they misinform customers.