it is best to divide the run in half with chicken fencing (I used the plastic kind from home depot, cheaper than wire) for the introduction. this lets the 2 groups see each other and get used to the idea before allowing them physical access to each other. the hens can also send messages to the chicks letting them know that the big hens are at the top of the pecking order without allowing them to actually hurt the chicks. my hens charge the fencing and try to peck any chicks who stick their head through.
I think it is best to give them 2 weeks in this divided run, then combine them. when you combine them, I would do it in a free-range area with lots of room to run away and objects to hide behind/under.