no experience, but relative had symptoms of precocious puberty.
if i were you, i would not be talking to the PCP about this. this is definately something a specialist will be more knowledgable. i would assume the growth scan is a good start but i would def want to see the blood tests as well.
dont waste time though, specialists usually have a bit of a wait(they waited a few months for an appt and we live in a very populated area where medical services are highly saturated). you want someone who already knows what they are looking for, as well as someone who is knowledgable of other issues without researching. lets say the tests come back looking like its not that, well then this doctor wont have a "runner up" to go and look at. a specialist will be familiar with other things possibly related that you or your PCP may never have considered.
aside from all this, concentrate first on precocious puberty. look at the more common first, and rule out as you go on. dont get yourself thinking something more troublesome right away when there is the possibility of other lesser possibilities. you know something is wrong, but just take it slow or you will be overwhelmed.
i wish you all the luck in finding an answer quickly. remember, you are your daughters advocate, so keep pushing until you feel you are getting the help you need. im sorry you have to be dealing with these concerns, its so hard with kids.