Ah okay, I missed the bit about mission 6.
The Melta stuff was personal preference - the role of Melta seemed to be eclipsed by advanced lasgun clips, which have both ToArmorPre and even more ToArmor. At the same range as the Melta, you can get more armor stripping with the advanced lasgun since it can fire more times to make up for the lower power and poor resistances vs. not vehicles, plus it's lighter, cheaper, and comes earlier in the tech tree. Thus, I wanted the Melta to be the clear winner in the armor-stripping role, so I gave it guaranteed armor damage by ToArmorPre but reduced the health damage to compensate.
Edit: This is one of the things I meant were unnecessary to include, as this mod started as some personal changes for the sake of my campaign.