I see the introduction of two pullet chicks as a positive, this can be achieved with sex-linked chicks to avoid unwanted cockerels (and the heartbreak you'd experience if you had to give him up or send him to freezer camp.)
If you ensure that you put the chicks in a wire cage in the coop so that the elder girls can see and hear these birds day in, day out, then incorporating them into a single flock when the chicks are about six weeks of age will go MUCH easier.
Dealing with more than one type of feed for one flock is nearly impossible. My recommendation is to switch your girls to non-medicated chick feed by adding some to your remaining supply of whatever brand of food they're enjoying now, with an eye towards having this old feed used up completely by the time your chicks are six weeks of age and incorporated into the flock. Put a calcium supplement beside their feeder. oyster shell or ground egg shell from your feed store will be sufficient. Your big girls will eat what they need of the calcium supplementation and later on the chicks might nibble a little, but will avoid overdosing themselves on it. Corid (amprolium) in everybody's water the first week that the chicks are on the ground (or in the grass), to bolster their immunity to cocci.