What age pullets to add to existing hen?

Due to a variety of events I now have only one hen. She's 21 weeks old and very healthy. It's time to add a couple more. Is it better to add 2 nine week old pullets or 2 14 week old pullets? Of course, this would be after a 30 day quarantine so they'd be older by then.

Also, once introduced what do you feed them? My hen is on layer crumble, but the new girls won't be old enough for that.

Thanks for your help!