Yes yes yes! This is very important.
The forum's Reply seems to have eaten all the content of "all of this" - maybe it only nests so deep. But I'll try to address some things.
Well, TL12 cloth went away in the new Core book, so that won't be a problem.
I think I've made stack limitations clearer (in general, you can't stack unless its explicitly stated) but I'm still editing myself.
For instance: diplovest does not explicitly stack; I thought about adding it, but since it's only a vest, then it doesn't make sense to stack it with better amour that covers more of the body, so I left it alone as non-stackable.
Also trying to deal Combat Arm armour by saying its for the arm only, but lets you use the arm like a shield.
Any armour that works as an environment suit or similar, regulating temperature, adding biometric sensors, urine collection, external radiators or other such things have little ability to stack, but available modifications to the armour can achieve the effect.
AP: As I mentioned early, Mercenary design rules and existing CSC items and the way various forms of armour piercing works makes it difficult to directly compare. For better or worse, this was NOT an exercise in redoing all Core and CSC weapons with Mercenary Field Guide rules. That would take a long time, potentially contradict many Core descriptions, and basically require duplicating a lot of that content to explain it. And likely give me a little bit of breakdown. I did 'fix' ADPS to lower damage by 1 Hit per damage dice (die?, dice?, neither one seems entirely correct, oh well). Low Pen is another thing. CSC says certain weapons are only half as good at penetrating armour and archaic armour is only half as good at dealing with more modern weapons. Low Pen multiplies armour effect for the weapon, so again, a different approach, complementary, but not necessarily compatible. If you're sniping with AP ammunition at troops in chainmail or firing shotgun pellets at the same, mileage may vary - that's what referees are for - pick an answer and use it consistently and most players will accept it. And if it turns into a mess, bring it up in the forum.
I can't predict (and have no knowledge) of the future, but the second edition of first Mongoose edition Mercenary presaged second edition Mongoose rules (say that three times fast). It's a bit of an evolution and comments in this forum on what works, what doesn't, and what is just plain confusing (like the previous sentence) is valuable input.