While I can’t comment on your question specifically (apologies!), I would like to point out that Marcus Aurelius’ policy here is hardly unusual. The demanding of surrendered enemies to serve in the army was a long-standing tradition by Marcus' time, and would continue into Late Antiquity. During the Principate, surrendered foes were grouped into units and graded as auxiliaries; in Late Antiquity, they were classified as foederati. Nitpicking your post a bit, if I may, these defeated Sarmatians would not have served in the legions, but rather in separate regiments classified as auxiliaries. In both instances, the principal difference between these new units and existing ones was their “ethnic” character (which likely reinforced unit cohesion). Certainly by the fourth century, there would have been few differences in terms of equipment and training between foederati and regular units. This unique “ethnic” character would have been quickly diluted, as new recruits were drawn from the area in which a unit was stationed. Thus in the case Marcus’ Sarmatian auxiliaries, upon transfer to Britain, new recruits would have been local Britons. Within a relatively short period of time, the only thing “Sarmatian” about these auxiliary units (or Gothic, or Frankish, or Vandal, in Late Antiquity) would have been the unit’s formal title.

Any and all info would be greatly appreciated. Much of what I find only relates to the possible relation they had with the Arthurian legends. While that in and of itself is fascinating, I'm interested in how a group of people would live in such an alien land.

I wonder why so many of them were sent to Britain? Were they badly needed at the time there?

could anyone suggest a marcus aurelius bio, either a biography or even just a book on his reign?

to be more specific, im wondering about his moral, ethical and religious sensibilities. so even if its just a chapter in a book, id like to read it if it can provide insight on this issue.