[Andrés]

An ISD should have somewhere upwards of 500,000 shielding points under my system, which definetly allows for lots of weapons fire.

What do shield points have to do with anything? The ability of absorbing lots of enemy fire?

If you're talking about tonnage, my estimation is that an ISD is about 51,000 kt, slightly larger than a B5 Omega Destroyer (a ship of similar size and "known" tonnage)
But to allow larger ships without hitting the max tonnage of 65,000 and make them more practical in SE4, I'm reducing their tonnage to 5001.

Shield points are roughly twice the hull rating, considering that average components have structure=tonnage plus armor and other more resistant components, hull rating of an ISD should be around 6000, and shields about 12000.

I don't want large weapons restricted to large ships. That's an unnatural restriction that I've never really liked. I'm restricting weapons placement by limiting supply generation per turn. You can put a massive weapon on a corvette, but it won't be able to fire many times, and it will hit other corvettes about 5-10% of the time.

I'm not fully convinced by the "supplies=energy" method used among others in the B5 mod. The game was designed for the "supplies=fuel" model and there are some ugly side effects in the "supplies=energy", like for example all ships having unlimited range.

The size restrictions I've made are not as heavy like in the stock game.
Just a little to prevent abuse and absurd designs.
And of course lighter mounts are still used in large ships when accuracy is the priority.

Many players have made mods to keep small ships useful until late in the game, but my worry is exactly opposite.
There is too little incentive to research and use large ships, even when the largest stock SE4 ships are ridiculously small compared with most sci-fi ships.

I've never seen a small ship carrying a single massive weapon anywhere in the starwars universe (hmm... maybe the Suncrusher, but that's a really special ship); why should it be so important to allow that?

From what you described, and my guess of the involved tonnages, your massive weapon wouldn't fit in a corvette anyway.