Since cats generally shed in spring and fall, and since some of them have the dandruff, too, I would suspect some sort of skin disease or allergy. But as long as they are getting a quality pet food, they should be getting the right vitamins and minerals. There are so many pros and cons about both tuna and egg I would not want to specifically say yes or no on either of them - my first instinct would be to take at least one of them to the vet (maybe the worst), explaining they all have the problem. Before that I might go to a good pet store (or just place a phone call to the vet, if you have that kind of relationship with him or her) and inquire if it might be a vitamin deficiency and try treating with supplements first. It could be, too, that your house is too hot and dry for them, now that winter is here. Humidifying the air might solve the problem. Hope some of these ideas help, and good luck!