Check for nitrite and nitrate. If nitrite is 0 and nitrate is high then your cycle is done.
You can hold off on buying the fish if you can. If not you can overfeed the Cory to keep the ammonia up for the bio filter. 1 Cory in a 55 gallon won't put out enough ammonia to keep a good amount of good bacteria in the filter. But if you are going to add more fish in the future make sure add them little by little to avoid mini cycle.