As far as bald spots I think some of that might be the feather change and some may be related to the pecking order they go through around three weeks and beyond. If they have any blood on them or they are unable to access water/food because of being picked on, take them out and isolate them , nearby if possible so they don't get too lonely. I know about this because I just had one of my almost 4 week old chicks have a bloody bottom yesterday and she had to be isolated but, sadly I can't have her near the others because of space issues but, I'm giving her lots of love. I'm not sure if a difference in breed makes a difference in their "order" but, some birds, from what I've read seem more aggressive, while some are docile and passive, best of luck, I hope this helps! I would make sure also that they have enough feed water and space for each bird 2 inches I believe otherwise this can also contribute to the aggession.