Ellen Ripley wasn't a marine. She was a freighter pilot. She lost her license after the first movie and resorted to work in the loading docks in a power loader (She has a class B license). Basically, she's nothing but a space warehouse worker.
It's also worth noting that Master Chief, if it is indeed a statement of rank and not some sort of nick name is actually in the Navy, presuming that the rank structure is based on the American system (and, given the game was made by Bungie I suspect this is indeed the case). In fact, Master Chief is the naval enlisted rank of E-7, making him a senior non-commissioned officer. The Seargent Major, however is likely a marine as Sergeant Major is in fact a marine E-9. Technically, the Sergeant Major outranks the Master Chief, which is odd as the Sergeant Major ever once attempts to give the Chief an order even though he is well within his rights (and probably more qualified than most of the fools giving orders). It's worth further noting that the Sergeant Major's position is one of "advisor", and is normally only found in higher echalons of command (Batallion and up). The highest ranekd enlisted man that purposefully takes the battlefield is that of First Sergeant - the senior NCO of a company sized element (true on both the army and marines).
The Master Chief as a E-7 would fill the role of senior NCO for a platoon sized element, whereas the E-6 rank is usually a squad leader and the E-5 rank (Sergeant) is the rank of a team leader.
Of course it's in space and a total war for the survival of the species is being waged so I suppose the rules may change.