>>/46031/
The ship would be huge to begin with, it's a battleship of course. Because it's so huge you could probably get the energy to use railguns though that still might not help a light ship stay balanced, you are still throwing mass out at speed and there is an equal and opposite reaction to everything.
They would have rapid firing cannons as secondaries.
The idea would be that they are so large that they have the ability to properly power and manoeuvrer themselves and not be subject to the whims of orbit.
I'm not talking about space combat with what we have now that is different and indeed what you say is correct regarding that, but you would not need heavy battleship grade guns for that and they would not be able to have armour on them anyway, what we use now is by necessity very small and light, you would not even need a cannon of any kind, somebody on the Chinese space station with an assault rifle could just fire a magazine of at the international space station as they were passing(if they even do pass) and it would completely wreck it, they are very fragile and things move at very high velocities in space. But what I refer to are proper battleships, the kind you would assemble in space itself or maybe on the moon so weight would not be an issue.