Mmmmh maybe the thread change its subject since my answer!? I don't understand why you choose to argue about an historical event over the gameplay feature this thread is about.This only happened after the Marian reforms.
Edit: Beaten. <_<
I gave an example to explain how some features and distinction can be implemented and explained.
The point was : only roman did this. It was a trait. Maybe tribe will have less manpower from some pop because it is how they work? Maybe another culture or gov can have manpower from slave? Maybe it is only after a tech that you will be able?