It is possible to do it like that, especially if the 'a' grades you start with are culls of a higher grade, like having ss parents.
If they are grade 'a' but parents are also grade 'a', it can take much culling and many years to go from grade 'a' to grade 'sss'.
+1. Parents genetics will play a huge roll. Babies I can see in my higher grade tank from 1 mother right now range from a few A/S grade,some SS grade, a few snow's and a few CBS as well. I'm sure those A/S grade babies when they grow up will throw out a few higher grades as their are only high grades in the breeding tank.
In my cull tank, I can see a few SS babies when it's mostly lower grades in there, so the babies in there will go to higher grade tank and the lower grades from the other tank will get moved to the lower grade tank.
20g platy, , 2 x 10g shrimp, 3 x 20g shrimp, 7.5g shrimp and 1 great dane/mastiff puppy.