Just to add to Carrie's excellent advice.
Aly, we had a similar problem with a young male tibbie who came to live with our neighbours. He came from a privileged background but his owners had to find a new home for him because of his behaviour problems which were just like Harley's. We found excellent advice in some articles about dominance & pets living together. We all consistently put these ideas into practice, & the tibbie is much, much better. At base, we found teaching him basic obedience 'Sit' to earn anything...like a pat or a treat...important. We also found that when he got 'bossy', it helped greatly to give him the 'down' command...because this means submission. He wouldn't willingly go 'down' , so we would say 'Sit' & then quickly pull his front legs forward with one hand & gently but firmly push his back down with the other hand.
But all the ideas in the articles were great.
Go to http://www.uwsp.edu/psych/dog/library.htm
Scroll down to (Potential) Behavior Problems
Under that click on Dominance/Leadership and More Than One Pet.
We found that working at these ideas consistently, over time, made a huge difference to the little dog.
All the best...& please keep us up-to-date.

[This message has been edited by Angels3 (edited March 22, 2001).]