As for Sam's ranking, I would think that in the military as long as she was a highly motivated soldier who trained day and night, and considering how small I would think the unit was, I would entertain the idea she could be the same rank as Riley.
Despite what you see in the movies about "battlefield commissions" and the like, promotion and military rank follow a very definite line of progression, and promotion to the next highest rank is determined primarily by four factors:
1) Time in rank. (You have to hold your current rank for a specific amount of time and demonstrate that you are capable of handling that amount of authority and responsibility before you are trusted with any more)
2) Recommendation ( Once you have demonstrated that you are capable of handling the duties associated with your current rank, your immediate superior and the Commanding Officer of your unit. along with whomever he chooses to contribute input, evaluate your performance and give their best estimate as to whether or not they believe that you can handle the duties connected with the higher rank)
3) Availability (There has to be a need for another soldier in the rank that you are being considered for. If there's no position available for a Sargent, you stay a Corporal, no matter how good a Corporal you are or how deserving you might be for the permotion on merit)
4) Location (This goes along with Availability above. Your current unit might not need that Sargent, but another one, somewhere else, might. In that case, you get the promotion, but you're relocated to your new duty station, which may be on the other side of the world. And accepting that promotion/transfer isn't a voluntary thing. You go where Uncle Sam sends you)
And even if you happen to be lucky enough to get the promotion and stay with your original unit, there is also:
5) Seniority (That means that the person who has held the rank longest is the one in command. This is to avoid the "all chiefs and no indians" situation that might otherwise arise.
So, you see, it would be totally impossible for Sam, no matter how good a soldier she might be, to rise to a rank equal to Riley's in that short a time. Therefore, if she IS his rank, then she is lying about how and when she joined the Army. And if she is lying about that relatively unimportant fact, what more important facts are she and Riley concealing?