SMB are not slow growers in general. They will grow substantially faster than literature says if they have plenty of the correct size forage and decent conditions to include water temp.
The smaller fish indicate they have ample forage for a 6-8" fish but how much of the lower end of the food chain are they removing such that few forage items grow through to feed larger SMB?
A 4 year old SMB on adequate forage should be more than 1.2lbs other than in the most Northern locations.
A lot of the culling process is dependent of what you goal is with this fishery. If large SMB is your goal, we might be in trouble already.
If lots of SMB is your goal, then start pulling a few when they hit 14-16".. You probably need to pull a few 4-10" fish every year depending on pond size. But remember, a 8-10" SMB is sexually mature in most cases so you always need a few.
It would really help a lot to know the size of your pond. Based on 15 original stockers I don't want to recommend something that isn't in line with your desired goal.
