It always seems to be such a hassle, but from many different "introductions" attempted,  I think putting a small pen within the bigger one (with the newer hens in the smaller one) for about a week, then take them out of the pen and put them in the hen house at night worked best for us.  They will still establish a pecking order, but not as aggressively.  This is the least abusive way we've tried, so far.  Good luck!