I don't know about the feeders being stunted to a limit of about 2 inches over a year's time. I have a few feeder fish from 3 different stores including walmart, petco, and an LFS. They all are growing pretty big. The oldest, about 15 months is around 7 inches and the smallest/youngest currently around 4 inches. Maybe I just got them from good stock but as the OP stated, the walmart and petco ones did come from tanks with a few bad fish. I quarantined them for a couple weeks first though. That's when I noticed a hole in the fish that I cured with salt.
Anyway, good luck to the OP. I really hope it all works out for you. If you get the chance get a bigger tank but if it works out for you then go for it.
Just when you ask for help about the fishes health that's always going to be comments you're going to get that they need more room. It helps the water quality for them to live in a bigger tank since they are messy fish, goldfish in nature are messy fish that is. And even with a filter the water can be dirty in other things we can't see and that will only change with more water, aka, bigger tank. I'm not preaching based on false knowledge. I have way too many goldfish in my 55g tank. They are going into an outdoor pond in a year though.
Anyway, good luck to the OP. I really hope it all works out for you. If you get the chance get a bigger tank but if it works out for you then go for it.
Just when you ask for help about the fishes health that's always going to be comments you're going to get that they need more room. It helps the water quality for them to live in a bigger tank since they are messy fish, goldfish in nature are messy fish that is. And even with a filter the water can be dirty in other things we can't see and that will only change with more water, aka, bigger tank. I'm not preaching based on false knowledge. I have way too many goldfish in my 55g tank. They are going into an outdoor pond in a year though.