So, the roman legion had four groups of infantry, Hastati and Princepes served in the front, Triarii defended the rear, and Velites skirmished from all positions.
My question is, how did Roman's decide during recruitment, who would serve in what position?
Boiled down, wealth and age are the determining factors. u/Iguana_on_a_stick covers the various troop types of the pre-Marian Roman army here and here (focused on the triarii).