Echo, of course you're bumbling! We all bumbled. We all did things and went, ooops, that was a bad idea. Or, hey, that worked out really well. Learning, it's all about learning. You are learning, cut yourself some slack!
I have found that mixing ages is a tense time and if you don't have A LOT of space, you can run into trouble! Sometimes it just isn't going to work and you have to be prepared to take steps to protect the safety of the young who can sometimes take a brutal beating. Sometimes the beatings end soon, other times they don't and you have to watch and step in when needed.
I also found a serious factor to watch for was the difference in bird personalities/temperaments. It has been my experience the better a bird lays, the more high strung and prone to viciousness she can be. Some of those big, dopey, not so great layer birds with the personality of a stoned hippy, they can take a terrible amount of abuse from the high strung, intense layers in the crowd. Your flock mix can often affect how peaceful or war like your hen house is. It might not be possible to keep a variety of different type birds peaceably.
Something else to keep you awake at night...roosters. Not by crowing, although that is a given. Right now I have my mystery white rooster with the blue/black hide. He is huge and I think gorgeous and I would like to get some chicks showing his size, stature and black skin. BUT..he's a complete idiot! He is so brutal and savage with the hens that he is about to have his head knocked off. When you enter your hen house mid-day and most of your hens are inside on the roosts or hanging out in nest boxes when they would normally be outside, note to self, you have a problem with a rooster. This guy is such an in-your-face a##hole, that most of the other chickens will go without food to avoid him. Despite my obvious attachment to his looks, his aggression and terrible bedside manner is a death sentence. An obnoxious rooster can destroy the peace in a hen house, be prepared to let go of boys who have no manners. (sorry, I realize you didn 't ask for any of this)
When my chicks are small they get chick crumbles, and when they are big enough to swallow a pellet without choking, they get lay pellets. That's it. No special feeding for different ages. Once they are past 5 or 6 weeks, lay pellets. It's a tough life. Except meat birds are always raised separately and given a grower diet.