Welcome to BYC forum......
Congratulations on being a new chicken owner.
It is pretty normal for older chickens to peck at chickens smaller than they are. And depending upon how much smaller the littler ones are, it could be that they need your help getting on the perch for the night--until they learn. --- Hopefully the pecking isn't hurting the littler ones, and they can get away as needed. IF any of the small ones get hurt...you will want to separate them right away. If chickens see blood they will peck a lot.
Usually you should only put birds together that are the same size.... With plenty of space and places to hide, I have seen a successful mix of birds of different ages...all penned up together.
You may have too few females for your rooster, so you may want to get more chickens... You don't need a rooster, remember for your hens to lay eggs..... The suggestion to need more chickens is pretty welcome to people on this forum.
Welcome and keep your questions coming. People here will be able to give you good answers. Congratulations on your new chickens.