Not sure what you're talking about. All damage modifiers are added together at the same time. So (1d6 -1, +1) is just 1d6 as the modifiers cancel each other out. In the example you quoted the die roll is 1, the modifiers cancel each other out, so the damage stays at 1. Cloth armour stops 1 hit, so the armour fails to penetrate and the poison has no effect.
The fire damage adds a point so the poison takes effect, right?