While it's good that you are planning on reducing the overstocking, it would have been better to not do it in the first place. Goldfish do the majority of their growing in their first two years, usually attaining well over 50% and up to 90% of their body mass during this period... if they are in a proper sized home. If they are in an undersized tank or overstocked tank, the hormone levels stay up too high and cause them to be stunted.
While your tank is OK in size, with 8 goldfish in it, the hormone levels would be high, UNLESS you did frequent PWC's (maybe 3 a week or every other day) or ran plenty of carbon or other more advanced chemical filtration and changed it out frequently. PWC's or chemical filtration are the only ways to remove/reduce the hormone levels which are one of the main causes of stunting.
Underfeeding them is the next major cause. So many sites say to only feed them once a day or every other day but when they are babies/juvis, they should be fed several times a day (some people feed them 6 times a day), which of course, requires frequent PWC's and filter maintenance to handle the waste created from all that feeding... but the goldfish will grow much larger (to their proper size) with proper feeding as babies/juvis.