I'm seeing that after reading all of this, the main point of the OP's can be summed up as:

"Why, on earth, when I fit a lower sized gun to a larger class vessel, I fit only one when realistically, including by scale and relative size, there's plenty of room for more than one turret?"

Disregarding the fact that the weapons are bonused in each hull in terms of being small/medium/large and extra large, thusly by design giving the players a good and strong suggestion that "they should fit these and as many as they can possibly can" when using that hull, the reality of the thing is that you often gain no bonuses (practically speaking) if you do fit smaller sized guns.

Ironically, this could be solved with basically introducing new weapons, that would consist of just lower classed weapons grouped together. with no difference in stats from the smaller guns otherthan being doubled up.

For instance:

You get yourself a Rokh. It can have, say, 8 rail guns. But you want 4 rail guns and you really want to be a anti frigate brawler, for some reason. Lelts assume that you don't simply do the obvious which is replace all guns with Blasters, and then slapping possibly a Haavy Neutralizer, and webs on your mid slots, and target painters and you know, the things that will make you hit a frigate.

Lets assume you go and start by fitting cruiser sized weaponery to your Rokh. Your turrets look silly and small, but you could potentially have two on tht slot, either by CCP allowing you to purchase and slap on two guns per slot, or but CCP making a "new" turret that consisted of "dual heavy electron blasters" which was in essence a turret with two electron blasters side by side, with all stats doubled except for tracking. Now lets assume you want even more tracking, and you fit say... Quad Light electron Blasters.

While the idea sound nice, and you get to scale up the DPS, you'd still make no bonuses, and there's the whole fact that you might never use those guns since you lack the speed to approach a frigate. But then there's another which is, the number of possible combinations fit. And there's also the annoying detail that they wouldnt' be covered by the battleship's bonuses.

I'd liketo see somethign like this, but I don't think it'll ever show up.