It's certainly good to chose birds of roughly the same size and age if you are going to add to your flock but it's best to do introductions slowly. Once you get your new girls home put them in a separate run and coop to your original birds but ensure that both groups can see each other. Keeping them separate (for a couple of weeks) is a good idea just incase your new girls have any infections, parasites or diseases that could be possibly be passed to your original birds. if you do not have a spare coop and run buy one. I know that sounds blunt but you will be able to justify it as you will at some time in the future need to separate one hen from the others be it due to illness, broodiness or accident.
If you are going to enlarge your current coop and run allow much more space than you think is acceptable. The more space you can spare the better, do not stint. Chickens get bored and frustrated and I'm quite convinced this can lead to behaviour problems like bulling, feather pecking and egg eating.
When you are ready to have the two groups together choose a dark night and put the new girls into the sleeping quarters calmly and quietly. Watch carefully the next day for squabbling and try not to intervene too quickly. If you do find one hen is being picked on try to make their environment as interesting as possible. Provide hiding places, jumping up places, interesting food and treats and try to feed everyone together but ensure there are several eating/drinking stations to so any picked on hens can find food and water. It can be quite a stressful time for both you and the birds but of course is perfectly doable. Best of luck.
OL