Mixing chicks of diff ages is not ideal, but it can be done. Some times I do it at night, in the dark, some times I introduce the chicks just one at a time. Below is a pic of a momma hen blending 2 groups, there is a 3 week old barred rock, 2 week old blue wyandottes and the yellow chick was 1 week old.
These 2 chicks were 1 week apart, the chick in front was new hatched, the one to the back had 5 other nest mates his size, I had them friend the 1 day old one at a time, and waited a day before putting the one day old in with the others.