Question: Soldiers tunic at the time of Trajan

[Tib. Gabinius]

It shouldnt be the question what most accept, more what would be more "authentic"
If you want to reenact an auxilia, you should know from "where" the men of it came and where they were placed, cause the auxilia show a bit of their own stile, in choice of the material like the style of the clothes.
For example the celtic and germanic troops had more clothes to get clear with the weather than the augusteian and tiberian romans, who had to addapt it step by step.

If you take a look on the trajans column, you ll see some auxilia cube men, just with "braccae" (trousers), shields and cubes, perhaps not auxilia, more numerii, but that show what is ment.
Different men, the fundatores, were shown in tunic and sagum.
Also we know about some sagitarii auxilia, that they worn their very long tunics (like the "kaftan")....