It's all semantics, anyway.  Only matters the roletype given for the damage bonuses.

Well what is your definition of a cruiser/destroyer in space anyways? They could have called it a "Cobalt Destroyer" instead of a light frigate and there would have been no difference.

Now, what I think you're wanting is a mod where the main difference of the ships is size, not what they counter like in Sins.  I.e. battleships have more guns that cruisers, but they use roughly the same weapons. In this case you'd likely want to look at the total conversion mods like Star Wars/Trek/Gate, since they follow their series canon and often change the game mechanics to be more like their universes, so the roles are less segregated compared to Sins.

If that's not what you're wanting, please explain what it is you don't like about vanilla Sins that would make you want a mod like this, so we can understand where you are coming from and point you in the right direction.

That still really doesn't tell me anything. What do you mean by "well designed"? Visually I think many of the Sins ships are well designed. I do find the basic Sins ship perhaps a bit lacking in diversity (only having 1 type of weapon?) but its easier to balance that way and I think it works fine overall. What exactly is the problem you want the mod to fix?