Old thread here.
Cerberus and their "manpower" actually get a fairly cool explanation down the road. It's just a pity that it doesn't get teased at all, in fact the opposite: everybody seems to think that Cerberus having that type of ability is perfectly normal.
US military is actually fairly unique in the sense that "marines" aren't simply a part of the navy. Shep is definitely a marine, just in a a military of that type.