I've fed chickens lots of expired bread in the past and not had a problem with it. I think if you offer some balanced feed, they'll be fine. Bread does have some protein, not enough but you can supplement as you said. When folks ask about feeding non-commercial things, my thought is always try it and see how it works for your flock. If you notice illness, lethargy, drop off in production, etc then it's probably not going to work, right? If everyone is active and glossy, and you have enough eggs, you're probably okay. Chickens ate a lot of things before commercial feed, they just didn't have the high production we expect today.