Why stop there?
Downscale marines and make them entirely personalized and customizable.
It shouldn't take that many trained professionals to take over a ship of poorly prepared, nonmilitary, probably mostly unarmed personnel, anyway.
So instead of hundreds, let's have marines counted in the dozens, starting at green rank, and let us customize their armor and weapons and training, and build them up over time. Think x-com (UFO: enemy unknown), but with the actual combat offscreen. Give them random names and let us nickname them, too.
Making us work specifically to outfit our marines in power armor and such to increase their survivability so that they make it to higher ranks would be pretty awesome, and I think it could be all be pulled off without detracting from the main gameplay at all. Also, extra cash sinks are never a bad thing, imo.