The problem with all pelleted diets is twofold: they are SUPPOSED to be complete. That having been said, if you add any vitamins and supplement with fresh foods, then you can cause over vitaminization (sp). Too much iron, for example.
Also, I cannot stress enough the need for good UV lighting. Some vitamins cannot be properly absorbed without it. It seems to me that MOST birds I've personally come in contact with do fine on pellets for a few years. However, after that time, I see more and more feather issues developing. Harrisons has a good reputation in the pet industry for lack of chemicals that are KNOWN hazzards, but still, a pellet is pellet. Most birds require more.
The added stress of "growing up" and not being able to reproduce compounded with a lot of other issues that constantly plague captive birds can sometimes cause stress related issues to appear. Feather plucking is classic.
Why not try what I've suggested for one month. Surely, you should see some noted improvement by then. Go to
www.landofvos.com and look around on that site. Though its geared for Ekkies, all birds will do well on the recommended diet. You will need to make sure that extra calcium is added to your birds diet for greys simply require more. You have nothing to lose. By switching this Congo to a totally fresh diet and giving her outside time, she has stopped plucking, totally. She is just now 3 years old.
THE OUTLAW