A lot depends on the age of the birds. If they are mature/adults, usually there is not much problem introducing a rooster to a flock of hens, especially if it is an experienced rooster. Sometimes the rooster is overly enthusiastic for a few days, if so I just separate him out for a while each day until things calm down. If they are adolescents, the rooster is likely to make a nuisance of himself and be overly rough on the pullets. If they are young, it is pretty much the same thing as introducing another pullet and all the potential pecking order problems.