You have not harmed them. People get so worried about feeding them and it’s pretty much worry for nothing. The only rule is do not feed Layer with its high percent calcium to growing chicks. Just about anything else goes. Depending on your goals there are some feeds better than others for certain chicks at certain ages, but that does not mean you have harmed them if you don’t feed the exact absolute best.
For example, for chicks that will become a laying flock some Starters with about 20% protein content say to feed it for 4 weeks, then switch to a 16% Grower. Other Starters say to switch at 8 weeks. The reality is switch when the bag of feed runs out any time after 4 weeks. Or continue feeding that Starter longer if you wish. Some people feed their chicks a 20% protein Starter/Grower combination forever, even after they start to lay. They just offer oyster shell on the side to take care of the calcium they need for egg shells.
Feeding a 16% Grower instead of a 20% Starter just means they don’t feather out and grow quite as fast at the start. They will still feather out and grow on a 16% Grower from the first.
I’m not exactly sure what a Cornish Giant is, I’m pretty sure it is a marketing name for the Cornish Cross. The general recommendation is to start them off at a high protein content and keep them at a fairly high content. By feeding them a 16% Grower for the first three weeks they will start off with weight gain slower. I don’t know what percent protein your Starter is, that information should be on the label, but if it is around 19% to 20% use it. Their final butcher weight may be a bit lower than it would have been if you had started them off with a higher protein feed, but it will still be impressive. You have not harmed them.
You don’t say where you are so I’m not sure what berries are in season, but go grab a bowl of vanilla ice cream and put strawberries, blueberries, or blackberries on it, whatever is in season and chill. Relax. You have not harmed them.
