Placing day-olds under a Broody

Hi guys,
One of my chickens has gone broody in the last couple of days (eggs are infertile since I don't have a rooster) and I wondering how long do I need to let her sit before I can try putting day olds under her. I recently had a broody that I placed fertile Araucana eggs under but only 2 out of the dozen hatched and there was a messy explosion of a rotten egg...so this time I'll try putting day olds under my new broody instead of eggs! The breeder that I bought my fertile eggs from regularly has day olds for sale but would I need to wait the 21days before I get some to put under my broody or can I do it after just a few days of her sitting with the same outcome? Thanks in advance!