I disagree simply because I have done it without any issues at all. I have 13 chicks in my brooder (all together) and they now range from 3 weeks to 6 weeks. We have the different age range because we were waiting for different breeds to come in here and there. I just added them in and they all seemed to do fine. Now, I will say that when I added my last chick who was a day old, I put him in a separate box for about a week with the next youngest ones, not the very oldest ones because it was only one little EE and I didn't want her to get hurt. That said, she was still put in with 2-3 week olds. Within a week I put them all together and they have done just fine. In fact, one of my 6-week-olds seems to mother the little EE and the EE snuggles under her.
Edited to add: If you see any bullying or too much picking on, put your new ones with some of your nicest older ones for a bit until they can take care of themselves. As Clucksworth said, there CAN be some picking but I think they are still young enough that you won't have a problem. I would expect to see it more with trying to add a day-old with 4-6 week old chicks.