If you are supplementing, is there a possibility of giving too much and creating other problems?
It depends on the supplements and the current diet. I think it boils down to some simple factors:
The MAJORITY of bird in captivity are actually significantly deficient.
What are you feed now? how long have you been feeding this?
I provide Egg food (which has crushed egg shell- and is a absolute MUST before breeding) fresh produce and sprouted seed.
On days when i do not provide this mixture (every other day) I use a supplement in the water or food.
I analyzed finches dietary patterns in the wild in comparison to show breeders in captivity. Both avian would increase dietary consumption during the reproductive cycle (from copulation to egg laying). However the finches in captivity who were provided with increased supplementation had the best results. You must condition your birds for at least 1 month prior to copulation- otherwise you risk death of your parents, and the young.
The most common cases of clutch issues are calcium, iron, fat, protein and iodine deficiency, which is very easy to correct. I feed a balanced diet, complete with crushed sunflower seeds, sesame, kelp, broccoli, apricot, cooked beans and dark leafy greens, etc. But furthermore in preparation for breeding, my finches receive Bird Builder by Harrisons in the water on days when they don’t receive specific whole foods (egg food, sprouted seed and produce). I also give multivitamin supplement in my Egg Food by Laberfers.
Furthermore, soft shells or low clutches is often a calcium deficiency- however this does not mean to supplement with calcium. Vitamin D3 is absorbed through natural sunlight through the skin, and it is required for birds (and humans) to process calcium. If you are providing crushed shells and cuttlebone, you need to increase the sunlight by a UV light or moving your cage to sunny area.
I would worry less about over supplementation and be more concerned with your daily dietary considerations months prior to the first egg being laid....
(my studies are more closely related to the finches.... but I have experience breeding very large species as well)