Roman Legion composition?

[Atilius]

A centurion must have had to detach his men into smaller groups

This does not imply the existence of an eight man unit of organization.

We also know that legionaries formed in lines four men deep.

Do we? Cite a reference for that.

But there are a few things we know about Roman Army besides what Polybus tells us. For example, we know a Legion commander or Legate had 6 Military Tribunes. What was their command?

In fact, almost the first thing Polybius tells us about the Roman army in his discussion of the Roman army (6.19) is that six military tribunes were posted to each legion. Later (6.34), he states that rotating pairs of tribunes shared command of the legion:

They [the Tribunes] divide themselves in twos, and each pair is on duty for two months out of six; they draw lots for their turns, and the pair on whom the lot falls takes the superintendence of all active operations. The prefects of the socii divide their duty in the same way. At daybreak the officers of the cavalry and the centurions muster at the tents of the Tribunes, while the Tribunes go to that of the Consul. He gives the necessary orders to the Tribunes, they to the cavalry officers and centurions, and these last pass them on to the rank and file as occasion may demand.

During this period, when it was extremely rare for a Consul to command more than two legions, the Consul had no need of a legate: he commanded his legions through the two tribunes currently in charge of each. It's also difficult to see how, if a Polybian legion were divided into ten cohorts, the command of the cohorts would have been distributed among the remaining four tribunes.