Unless I've just not done enough digging (which is possible), I haven't seen much on how officers are selected in the legion. Is it safe to assume that it parallels the roman legions, with most of its officers being direct commissions from the upper class or more politically connected? Or are it's officers actually selected based on a more meritorious system, where even the generals were once simple foot soldiers?

At least in Morrowind, the latter. It's probably a bit political in Cyrodiil or High Rock, though.