Rapid growth is not good for any mammal and since the day I started talking to ranchers, I have always been told that you do not want a fast growing chinchilla. I had one kit that hit 680g at 5 months and by 10 months and 960g, he had developed teeth problems. He was growing normally until he hit about 4 months, after which he gained almost 200g in a one month period. Previous litters from the parents grew at normal rates and had no problems and this male even had two littermates that grew at normal rates and have no teeth issues at almost 3 years now. I also do not feed any kind of supplement, my chins get daily pellets and hay once a week. I've always been told by ranchers that if a chin grows too quickly before 6 months, don't breed it because it will have teeth issues.
I think we will see more instances for these rapid growing chins that develop teeth issues early on as people push harder for larger size at younger ages. It's not natural or healthy for young animals to be so large. Size can be attained through careful breeding of healthy, larger animals...but all too often I am seeing supplements pushed or high protein diets as an easy "fix" for smaller animals. I stay away from supplements and it hurts me on the show table at times, but I feel my animals are healthier and it's much easier to spot problems in my herd.