From what I've heard around, placing the newbies with the others at night after they've all gone to sleep works the best. From my experience, I add my newbies to the flock and give them two weeks to settle the new pecking order. During that time, I try to prevent the newbies from hiding because it will take them longer to adjust.
As far as the rooster is concerned, has your two-year-olds ever had a rooster before, and if so, how long ago was it? They might not like him because 1, they're not used to a rooster, or 2, they don't like his inexperience.