old and new chicks

Hi i seem to be asking loads of questions at the moment, but thats how you learn, so hope you don`t mind!
 just wondering if you can keep 3 weeks old chicks with newly hatched chicks or if i`d need a seperate brooder, so far out of 7 eggs 5 were infertile and 2 have chicks in , so want to get some more eggs in the incubator as soon as these, fingers crossed, hatch, any advice would be greatly appreciated, thanks.

I hatch constantly but not always large numbers.... I have one brooder for the very young birds with an electric hen ... all the new ones go in there, and at the same time I take out the largest of the existing chicks..... the older ones show the younger ones how to drink and eat

I haven't had any problems and have only lost one chick which was a tiny serama who didnt look strong enough in the first place

Once they are a little bigger I put them in the top half of a giant rabbit hutch with a home made leccy hen.