Integration is a difficult topic.  There are so many variables that you cannot come up with one answer that is right for everyone.  I've had no problems integrating 12 week old brooder raised chicks with my flock, but I free range mine and have plenty of room in the coop.  I've had broodies raise chicks with the flock and wean them at seven weeks.  The chicks make it OK with the flock without Mama's further protection.  Mama took care of the integration issues while they were young, but the chicks still have the pecking order issues to work out on their own.  They usually solve that by running away from any adult chicken until they have the size and, more important, maturity to challenge them.  I have lots of room during the day and lots of room in the coop where they are locked up at night.  The chicks can stay away from the older chickens.  That makes a lot of difference.  
I also have a pretty laid back flock.  I've seen 2 week old chicks leave Mama's protection and go eat out of a feeder with older hens eating right beside them.  Sometimes the older hens ignore the chicks and sometimes they give them a peck to remind them that according to chicken etiquette, it is bad manners to eat with your betters.  When they get pecked, the chicks make haste back to Mama's comfort and protection.
There are different kinds of pecking too.  There is the "I'm going all out to kill you" pecking that is a real danger to the chicks.  Some hens have the meanness that they will try to kill other weaker chickens, whether chicks or just weaker.  Then there are the "Mind you manners" type pecks.  These are pretty hard and are intended to really get the chicks attention, but they are not an attempt to kill the chick.  
Integration is a dangerous time.  The chicks have to earn their way into the flock.  There are some tricks and methods that ease the transition, but there is always a chance that it can go deadly wrong.  I think space is a critical, but personality of the individuals is also very important.  I've never had a problem with a rooster threatening the chicks, at least a dominant rooster.  Non-dominant roosters don't have the same mindset that the dominant rooster has.  The non-dominant are more like the hens that pecking order is important so they could be a danger to chicks.  And not all dominant roosters have the mindset to protect all members of their flock that a good dominant rooster will have.  I don't know if my dominant roosters' behavior is typical or if I have just been lucky with all of them. 
When feeding chicks and layers together, I feed them all grower with oyster shell on the side.  The chicks may eat some oyster shell, but not all that much.  The laying hens will eat what they need.