I've read a lot of varied replies on this topic, so it seems there really isn't a 'right' or 'wrong' answer for this. I don't claim to be a chicken expert (though I'm logging enough hours on this board to learn just enough to be dangerous), but I can tell you what I've done/experienced.
I did things a little different, not because I felt like doing things my own way, I really tried to read all I could and do things the 'right' way, but I wasn't able to buy all of my chicks at once, only had room for one brooder, and the chicks were clamoring to get out of the brooder, escaping and throwing poop and shavings EVERYWHERE!!!
So, I started 'experimenting' while watching my chicks for any sign of discomfort and waiting for some more experienced chicken keepers to come up and smack me for my temerity.
Sort of by necessity my flock is a crazy mixture of ages and sizes and I have had no issues (well I have lots of 'issues,' but I haven't lost any chicks over the mixed ages thing).
Basically, what happened was that I bought two chicks at the feed store, then went back a week later when they got their next shipment and got a few more, and the week after that, and so on.
Every time I got some more chicks I added them in and watched to make sure everyone got along. Now I have most of my chicks out in the coop (ages 3 wks to 12 wks) and everybody gets along fine. Inside I have some 1 wks olds in a brooder that I just received from MPC and wanted to keep under a heat lamp for a while til they grew some feathers.
Yep, I not only broke the rule about mixing ages, but I didn't keep them under the heat lamp for 5 or 6 wks either.
The first group was in the brooder for quite a while, but it became overwhelming when they got bigger and made such a mess indoors so I cautiously put them out in the coop and watched to see if they seemed cold. It's 99+ daytime, 75 nighttime, and they all cuddle at night so everyone did just fine. I even added some 1 - 2 wk old RIRs into the coop because they were feisty and kept some BOs of the same age in the house in the brooder and the RIRs have grown 3 times the feathers that the BOs have!
I would recommend, though, that if you change anything, add chicks, whatever, that you do it when you have plenty of time to observe and make sure they are going to be okay.
The only time I ever had an issue was when I added a tiny little modern game chick named Boo-boo, and the bigger chicks apparently thought her toes were mealworms. One of the andalusians snatched her up by her toe and ran around peeping while the others chased them around! Boo-boo just hung there upside down screaming her head off!
Luckily I was there and waiting to see if the little guy could fend for himself so I was able to put a stop to it and move him back to the brooder with some younger, ahem, non-hooligan chicks. Boo-boo was non-the-worse for wear, but I'm going to hold back on adding her to the flock until she's big enough not to be mistaken for a chicken-treat!
