You should not attempt to put them together until they are about the same size. Even with that, I always introduce any new members to the flock via a wire cage that houses the new flock members. After 2-4 weeks of them being around each other, then I will attempt to introduce them together. It's worked so far. But my first 3 hens are by far the leaders of the flock. They chase everyone outside in the mornings, and they round them all up and back into the coop at night. It's really cute to watch.
As for your little rooster, try putting him into a large wire cage inside the coop with another flock member for a few weeks. Then re-introduce them both back to the flock at the same time. The purpose of the large wire cage is so they can see eachother and get used to eachother with out the issues of confrontations, fights, or injuries. If you put a current flock member in the cage with him, they will see him as less threatening since one of their own has accepted him.
Hope this helps. Keep us posted.