10 year old westie playing violently

Question
    Bruno, our 10-year-old Westie, has had free reign of our house for all of his life, and has always been a nonviolent and loving dog. Recently, a relative who had 2 young female Westies became unable to take care of his dogs, and we decided to take them in.
    The girls are great, well-behaved, housetrained dogs, but the problem comes when we let Bruno and the girls socialize. Most of the time, they keep to themselves, but sometimes Bruno calmly walks up to one of the girls and, it looks to me like he tries to bite her on the back of the neck or something. The whole time, they're all wagging their tails and it looks like nothing's wrong, but it just seems violent, what with the barking and growling.
    Are we doing something wrong? What should we do when they start this kind of playing?

Answer
If they are all still wagging their tails and not really snarling and showing their teeth, they are probably playing.  Dogs have wierd ways of playing and it really can seem like they are fighting.   
Have they all been spayed and neutered?  If not, they could be asserting their dominance.
If you are afraid this could escalate you should firmly tell them NO when they do this or do something to distract them and divert their attention.  This may take a little work to get them to pay attention to you but if when you do get their attention you give them a treat, they will learn that it pays to stop and listen to you.