Ammunition sounds a bit more like a mod thing, tbh. If you start with ammunition you would need go over to fuel, life support, etc...etc. I tottaly like a simulation feeling but i think it´s not what the game itself intended to be (a hardcore simulation).
Why not just use the ways we got allready to determin the size/useage of turrets. Okay i agree the upgradeslots may not the best solution. But why not use the same like we use it on hyperdrive and more upgrade slots allready? A C&C block. Or more like a Firecontrol block. That block would become part of a ship and increase it´s turret slots. So basicly a firecontrol would be a operator room where all the gunners sitting, controlling and checking whats goin on.
But i would also try to limit what kind of guns u use. As example you got a 1000 ton ship and put a 600 ton gun on it, like a big turret. Now you fire it. In that case i would not only give that size of a gun a big recoil but also damage the structure of a ship. If you just scale a gun way to big for your ship you will have to hurt yourself to fire it. I think that combination of a extra block for more turrets and a limiting factor that is calculated out of the size of your ship would be a good way to balance it out so you cannot build a giant glass cannon and smaller ship still got chances against bigger ones. And of course, the bigger, the slower. So it would be:
Size = Recoilforce (needs energy/thruster to compensate) + Firingspeed + Extramass
I think that way you could still get big guns but not overpowering your ship after all. A smaller ship could still carry more smaller guns/turrets and still use them propably. The Recoil of the big turrets would allready make it very hard to hit small ships with a bigger gun. Count in slower moving rate of a big ship + the out of balance mass that makes it hard for just fly in circles full speed and shooting and actually hit the target.
On the other hand, a fixed mounted gun with that special no rotation block could make it easyer to handle a big gun by absorbing more of the recoil and reduce structurel stress but removes the turret capacity =)