1 week old chicks with 3 day old chicks?

I have ten, four day old chicks in with three, week to week and a half old chicks.  So far no issues, but it's only been 5 hours or so.  Two of the younger ones are bantams as well, so I am keeping a really close eye on what happens in the brooder.  I are prepared to make another brooder if need be, honestly I'll probably have to once they get larger anyway. 

I'd just go for it, I feel like now is the time and a week difference is better than months later when you decide you really do want more chickens.  I could be way off though, hopefully someone else chimes in.

i think you will be fine at that age. i have 6 that are 2 weeks and 2 days, 5 that are 1 week and 2 days, and 6 that are 6 days all in the same brooder and its one bi snuggle pile!

chickens are pretty docile at that age you should be fine

They will be fine,  but sometimes they can be bullies, pecking each other !   you will have to watch them   to make sure they dont over do it !!

For what it's worth I have no issues.  The Red is a bit peckish sometimes but not damaging, she is just clearly the largest and the rest seem to really get along.  Actually the young BOs are the more rambunctious ones.  The have their favorites; the bantams actually split up, one likes a group of BOs, the other is very fond of one of the older EE. 

Good luck

Last year I brooded chicks that were a total of 2 weeks apart and had no problems at all.