A behaviorist may be a good idea, but I would also consider obedience as well, maybe first. We are going through some similar issues with our new rescue, and we are starting classes this weekend. I went throught classes with our Toy Fox Terrier, Chester 4 years ago and I think it made all the difference. Sometimes too, nipping starts out as play and does escalate, sometimes it doesn't. Chester still barks and growls to get us to play with him, but when we tell him to stop, he does. I personally would get a handle on it asap. Good luck and let us know how things go.