Would you be mixing them together in a brooder or wanting to put the chicks from ideal under the broodys? I would be very cautious about mixing ages under a broody, a mother of 20+ day olds is unlikely to accept day olds.
In the brooder, some people will say no some will say yes. I would say yes! I've got 3 week olds, 9 day olds and day olds all together in the same brooder with no problems.
I just introduced on neutral territory (a big cardboard box!) with a big bowl of food as a distraction while I was cleaning out the brooder. I reorganised stuff in the brooder to, so when they went it it looked different for the older ones. I kept a close eye but all are fine.
The little ones will snuggle under the big ones and the big ones show the day old show the day olds how to eat and drink. Just make sure you have enough feeders/drinkers so no one gets pushed about.
Here's my brooder (they are having a size upgrade tomorrow!)
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View attachment 1421505 Podge wanted her close up!
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Tiny fluffy brahma chick