Too much of anything can be bad. And only you can gauge just how much is too much, based on the dogs tolerance to the food and his reactions to it.
As far as people food goes, I know to "never" give them onions;grapes and chocolate. I know there are more out there, but I can't pull them off the top of my head right now.
CEREAL isn't a good idea to feed your dogs imo. I used to do cereal and bananas every morning for my dogs and have stopped after speaking w/ my nutritionalist. I have an article, I've gotta dig it up when I get home, that explains just why. I know it has something to do w/ starch being broken down into sugar when digested by the body and how it effects smaller breeds much more significantly than it does larger breeds just because of the motabolism and size of the organs. There's more, but again I'd need to dig it up.
Dogs really weren't designed to digest grains any way or lactose. Not like we do anyway. It's a filler WE incorporated into their diet because we know it's good for us, well guess what? Or dogs don't really need it. *anyway* maybe more later.
I don't want to continue sounding like I'm talking out my rear w/o being able to back it up! (no pun intended)