A lot of this depends on how hard you want to work and your set-up. There are risks when you integrate new chickens. Some of us have greater risks than others. Your space gives yo a big advantage over a lot if people to try it.
I'd approach it one of two different ways. In either case, set up a seaparate coop for the small ones and any full size ones you want to start with them. I'm assuming chicks. Keep them separate from the others until you are ready to free range them. Then you could try just letting them go during the day but let them sleep in their own coop at night. There is a fair chance they would be able to merge without huge problems, but they will remain two different flocks until the young ones fully mature.
The other option is to keep them separate and gradually introduce the old flock to them. Take one or two of the milder birds in your existing flock and lock them with the new birds when you think they are old enough. When things settle, introduce a few more. Eventually, you should get them all merged.
I'm not going tio guarantee either way will work, but I think you have a reasonable shot at it.
When a hen mates with a rooster, she squats. This spreads her on the ground and distributes the roosters's weight. If she tries to stay on her legs, she is much more likely to get hurt. Think of it as a car tire rolling over a wide flat rock versus a car tire rolling over a sharp nail. One way is a lot more likely to cause damage than the other.
This is not to say that there won't ever be problems, but a larger rooster mating a smaller hen is often not as risky as you would think, especially if the hen does her part and squats.
