Currently Armor value cannot go below 0. I tested different ways (set it <0 at start, via same stacking effects, via different effects and so on...) but armor couldn't be reduced below zero. In most time it is great, however I think it could be good feature request to allow negative armor. If unit has let's say -2 armor, then every attack deals 2 additional damage (would boost rapid-fire enemies attack nicely).
Best would be if modder could choose if he wants that armor could be negative. Either it would be set on techtree <allow-negative-armor="true/false"/> or even better allow it to be set with effects. Currently there is a flag stating <allow-negative-speed />. How about if there could be also <allow-negative-armor="true/false"/> included via the effect? So that if you would like the effect just reduce armor, but not set it negative (as it is now), it is fine. But if you wan't special "corrosive" effect, just like Devourer has in StarCraft I, which would eventually reduce the armor below 0, it could be great.
