How old is too old to mix baby chicks?

[Mistols]

We just picked up 8 chicks at the store on Monday- 2 are a little older. I can tell by size. But most are like a week old. They were out of the Sussex that my son wanted and won't get any until May 5th. Is that a bad idea to get a new chick and mix her in with chicks that are almost a month old? Or am I over thinking this?

 

[Fred's Hens]

That is stretching it. You will have to have lots of room and keep a very close eye. I might prepare a brooder that is divided with a screen, allowing the little ones the ability to see the older chicks. They are social and like company.