I asked this same question the other day.
I have 6 2 week olds and I just bought 6 almost one week olds (but they are all smaller types of chickens) The farm I bought the newbies from said that I should go ahead an mix them now cause it's easier. So I did. I had been planning on moving the older gals into a bigger brooder, so I set that up and put the new chicks in first, let them get all cozy and then added my older girls. Everything seemed to go smoothly. But now I'm noticing that the older girls are picking on each other....well 2 of them are picking on the others. I have been keeping a eye out for the littles and they don't seem to be getting bothered, all though I am worried they will get trampled!
I just put them together yesterday so I'm going to see how the next 12 hours goes.
I'd love to know how it works out for you.