They can mix, you just need to let your bantams grow up a bit. They are still very young and would probably be bullied by your older hens. Let them get well and truly into lay, used to other hens and then try and move them. It is always much better to have a full hen house in winter - they keep each other warm.
you are best to slightly alter the set up of your house and run, make lots of hidey places and put multiple feed points and drinkers down. thoroughly clean the house (if poss move it, even if it is a matter of ft) and if you can, exclude the two older hens to start with. Let the younger bantams find their feet, then pop your older ones back in. You want to avoid the older ones getting territorial.