Questions about Turkic people in the Tang dynasty

[mr friendly guy]

Its well known that during the Tang dynasty China experienced an influx of Turkic people. Some of them played big roles in history, such as An Lushan (yes I know he is of mixed heritage). So I have a few questions.

1. How large of the empire's population was made up of Turkic people. Contrast to say the Han chinese population. Obviously it depends on which period, but I was thinking in terms of either at the dynasty's height, or around the time just prior to the An Lushan rebellion.

2. What were their contributions during the entire period of the Tang dynasty - cultural, political, military etc. Any famous examples?

3. And this has sort of bugged me, but what did they look like? Dress, fashion, physical features etc?
Bear with me. Modern Turkic people vary between the central Asian states and Turkey, so I don't think looking at one of the modern Turkic people is necessarily going to help. You might say, so what? Why should you care what they look like? Its just that in Chinese historical fiction epics, they just have a Han Chinese actor, do their hair in a different manner (presumably the style of the people) with different clothes, and there you have An Lushan. I am just wondering how accurate this representation is.

Any help or any references to look up would be appreciated.