As said in both replies, you are definitely able to work with an outside agent if you buy through Toll Brothers. To be honest, I considered applying to work for the company, and in doing so, I learned a bit about how they do things. If you get an agent through Toll Brothers (which is very easy to do -- you just have to go into the front office and tell them you are interested in a house of theirs), you have to understand that they are not working in your best interest. They're looking to get the best deal for the company. However, if you have an outside agent of your own, they can bargain. In theory, you could bargain as well, but it might not be so simple. Thus, having an agent working for YOU would be in your best interest.
Hope that helps!