Thank you Vortigern and Cearas (and Cearas, please do give more information, it's always appreciated)! :)

Originally I actually was looking for some (modern) studies in this subject written by (modern) scholars, not the ancient authors' comments themselves. But it doesn't matter, the extracts you gave were so good that I can use them, it's perhaps even better in this way (hail the original sources!)

However, if you happen to know some historical researchs, do not hesitate to advertise them. (I've already read H.D.Rankin's "Celts and the Classical World", but it didn't give me my answers.)