There's a reference in Polybius (I believe, can check in the morning), that discusses Romans bringing in captured Carthaginian mercenaries in the First Punic War. In this, the Romans were said to have made note of physical attributes that the individuals had and put them in different roles depending on that. We do have to be careful about taking this as gospel for a couple of reasons, but it does at least tell us that Polybius didn't view this as an abnormal practice or out of the realm of possibility (or even his imagination). We have some Roman infantry specialists around here so hopefully someone can comment on the possible practice further.