I'm going OT a bit, so bear with me while we swing back around to it, it has to do with the inspiration for the idea.
My real life experience in aerospace is actually building aircraft, I tend to think, in real world applications, and not in the math of the game. So when I see "reinforced alloy, or military grade composites" bulkheads, my brain visuaises that as extra material being applied to the bulkheads, skin, structure, etc of a spacecraft, for additional torsional, compression, and tensile strengths. That's my mental interpretation that provides some immersion into the game. I know in game it's a bunch of math, not an actual change to the ship, not one single polygon changes. Similar to how the polygons we can choose to add, have no effect on the performance values of the ship. Everything is a modification of a value that is referenced for the ships performance. But in my head, I add that module, and I can see it.
That all said, I wasn't thinking of a negative number being input into the game, on something that has a value of 0. I was thinking of something that adjusted the base value of the ships mass. Again, this is my head thinking in real world experience. If you reinforce the bulkhead on an aircraft, that becomes the weight of the airframe. Let's say you replace a bulkhead, for example, one made of thick solid aluminum, with a composite, either kevlar, carbon, or blend there of, whatever you like. You get a weight reduction, some some change of torsional rigidity, but increased tensile strength, and similar or better compressive values. That was the idea behind, lighter, but brittle.
After your explanation, it clicked in my head that I have never seen the base value of mass on the ship change. The bulkhead additions change, the total mass, that is reduced or added to, adjusted by type and or engineering with a base module value (lightweight alloy) of 0 as explained. So my brain was mistakenly on the physical track of, lighter spaceframe components, replacing heavier components in the spaceframe, causing a reduction to the mass base value of the spacecraft. -x% of base hull mass. After what you taught me, I can see the problem. The game treats bulkhead reinforcements as a module, just like any other module. The lowest value the engineering can handle in the current system is 0T as you said, due to some values expecting a number zero, or greater than.
What if we duplicated the lightweight alloys module, pre-engineered it weight reduction x% and rebadged it as Lightweight composites. Then you could have the option for one or two, completely separate, Lightweight composites only, engineering upgrades? Of course ultra light would be the goal of one of them. Then your math could be corrected to -10T + (-10T * -10%) = -9T for negative affect, and -10T + (-10T * 10%) = -11T for positive affect. With no need to bother the other formulas. Again, we are targeting a different crowd, so kinetic, thermal, and blast resistant are useless for this proposed module anyway, and could just not be an option for it.