Since these are new mama's I would keep a close eye on them too. If you can possibly go out real early in the morning while still dark and slip the babies under her/them at that time it may be better. If she does not accept them you will have a chance to rescue them before the get chilled.
I have had sucess giving a few extra chicks to a mama with other chicks near the same age but I have also had some reject any additional babies. Just last week a Sizzle hen kept them warm all night but I found them later in the morning hiding behind the water jug. When I moved them back to the mama, she pecked them and they took off running again. In short you need to watch carefully especially with newbies.