I have 21 chicks around 4 - 6 day old chicks currently living in an indoor brooder.
Some broody hens will accept practically any chick, but some will not. The chicks themselves need to imprint on the hen, otherwise they might not accept her as their mother. This usually works better if the chicks are 3 days old or younger, but there have been times it has worked. Sometimes the broody hens have to bully the chicks into accepting their authority. This can mean pecking. I consider the chick's age to be less than ideal but you can still certainly try. It can work.
A bantam cochin and a silke mix would be able to cover 10-11 bantam chicks each, wouldn't they?
Different bantams come in different sizes. Silkies and Cochin are not generally tiny bantams so they should be able to cover them when they are baby chicks. But even bantam chicks grow. By a couple of weeks they may have trouble covering all of them. But by a few weeks the chicks are capable of handling colder weather. It's something I'd watch. When I give a hen eggs that will be hatching in colder weather I give her a few less than normal.
And it's best to give a broody chicks when it's dark, right?
That's the way I do it. I think it helps my chances. But often a hen will take them in the middle of the day. with living animals you never know.
Should I make sure both have some eggs under before I give them chicks just so the chicks wouldn't appear from thin air?
I have eggs under mine before the chicks get there to keep them interested in being broody, not to try to trick the hens. I don't know how necessary that is but it feels right.
One has been broody for a couple weeks. I think around 2 weeks. The other about 2 days. The one that's been broody 2 weeks is a first time broody. The other has been broody before, but hasn't hatched chicks because it was the middle of winter.
I personally do not put any faith in that first time broody stuff. I've had success with first time broody hens and failures with experienced broody hens. I've had very little experience with hens that have only been broody a few days, mainly because of the way I manage them. I did have a hen that had been broody 2 days fight and take chicks away from a hen that had just hatched them. I don't know how important how long a hen has been broody really is, I suspect it depends on the broody, but I am absolutely sure that if you don't try it won't work.
Alright well if I can't give the chicks to a broody would they be fine in that temperature with a heat panel? I could set up a brooder out in the coop.
My brooder is in the coop. If you can keep an area warm enough that can certainly work.