I've been playing around with building defences against boarding, and guards do not seem to be the way to go; regular Air marines seem to easily cut their way through a similar number of guards. Because guards are not available for boarding, they could wear heavier armour than the boarders (taking less damage), and carry heavier guns also. Guard stations might be able to give a boost to defenders, such as: better defender cover so less likely to be shot, impeding invader movement, heavy mounted weapons (internal gatling guns, anyone? It might even be neat to have a new defence post, a hard point, where several defenders could benefit from these bonuses simultaneously.
Hmm, do note that the guards are a lot cheaper than the marines.
True, but their quarters use more mass and space per marine, both of which cost a premium for an airship (but is ideal for towers). I don't mind that guards are the weakest marines, but I would like to be able to build ships that are difficult to board without also being excellent at boarding. Maybe an elite guard of some kind could be considered?