when can i mix diferent age chicks

Hi i have 1, 2 week old chick in a brooder and more eggs in the incubator due to hatch in 10 days, at what age can i mix my chick with the ones about to hatch, i`m also getting a next batch of eggs sorted which will be a mixed bunch of 12 and 6 polish frizzle bantams, would these need separating or will they be ok together(when they hatch), just needing to know how many brooders i`m going to have to have ready, thanks

Don't know about Polish, but last year when we tried breeding our own purebreeds, I had 4 lots of chicks hatched a week apart.  Brian divided my small brooder shed into 4 pens, then after 4 weeks and the chicks were bigger, I took down the partition between the oldest 2 pens and the week after the partition between the youngest 2 pens and a week later the last partition so they were all together.