That missile turret isn't going to do squat against anything. That won't take down most peoples' drones, let alone fighters. Don't even think about it harming capital vessels.
It's far too small for the task you want it perform.
Basic turret design says that you make a base. It contains rail mass enhancers (If necessary) and possibly shields. The barrel should have the weapons systems and be capable of whatever amount of rotation you want. Do not add multiple barrels. Don't bother with additional turrets on top of existing turrets. There's no point, it just creates more lag and glitches. Make sure that all your turrets (Except for the very largest meant to destroy ships of your own size) have good firing arcs---if this ship becomes as big as you claim it's going to be, you are gonna be getting outmaneuvered by vessels with half a million mass....which would hurt if you don't have adequate turrets.