New chicks older chickens

Can I get one of my older chickens or ducks to take care of the new chicks we just bought? Taking care of them in the sense of keeping them warm and whatever else a momma chicken might do?

Maybe? I've heard of people who stuck a chick or duckling under a boody chicken/duck and broody interpreted the chick/duckling as a baby and happily adopted it. So, if you've got a broody duck/chicken, you might be able to swing it. Keep a really close eye to make sure there's no aggressive actions on the part of the "mama."

Also, I've heard it's find to have a chicken raise ducklings, but not a duck to raise chicks, as ducks swim and will try to make the chicks swim, and it just doesn't end well...

I think Nicole summed it up nicely. From what I've read, it's only maybe going to work if the chicken is already broody, and the stealthy addition is done at night when they're resting. I've always had to sequester new chicks from my older chickens because they bully baby chicks in a bad way.

The older birds are harsh on young ones even after they have some size & all their feathers. I suggest keeping them apart at least until then unless you have just the right brood hen.