\"Celtic\" military technology and the Romans

[Youngster]

Those do not look like pteruges to me, is there any other evidence for Romans using mail to cover places plates couldn't?

Please correct me if I am way off; I partly post to learn if my conclusions/opinions on one side or another is right or wrong.

I'm quite certain that those are pteruges. It's a bit stylised, but I think it is fairly clear.
I've seen no evidence that mail was ever used in conjunction with segmentata. It would have made sense to use something like medieval voiders to protect the armpit, but there is just no evidence for it. There is however, some examples of plates that were probably used like a small pectoral over the chest of later era mail shirts. Many modern illustrators also suggest that late era cataphracts may have worn mail shirts beneath scale or lamellar cuirasses.