Also, does anyone have any experience with a house rule of "armor does not stack - only use the best armor item you have, ignore all the rest."? Obviously the special rules for the armored suits and whatnot would still stand from arsenal otherwise they'd be useless, but I think you know what I'm getting at.
I'm assuming you mean "armor that the books say are cumulative", because, as it currently stands, the rule is you only use the best armor (plus those bits of armor that specifically are cumulative).
You'll notice I'm carefully using the word cumulative because Shadowrun, to be different from everyone else, uses the word "stack" to refer to the exact effect you mention, two pieces of armor worn together with only the best rating applying.
If a character is wearing more than one piece of armor at a time, only the highest value (for either Ballistic or Impact) applies. Note that some armor items, like helmets and shields, provide a modifier to the worn armor rating and so do not count as stacked armor.
Included in that list of thing-gummies which provide a cumulative effect are the PPP system, Armor Spell, adept's Mystic Armor, natural armor (like that of trolls) and so forth.
However, a guy wearing a Chain Shirt (2/7, Arsenal, pg. 48) and Armored Clothing (4/0, SR4A, pg. 327) has an effective armor rating of 4/7. If he then put on a Helmet (+1/+2, SR4A, pg. 327) his effective armor would now be 5/9.
As for getting more restrictive than that, usually their body ought to come into play sooner or later. If you allow the custom-fitted armor optional rule, I could see armor ratings getting pretty astronomical, but aside from that, -1 to both AGI and REA for every 2 points of armor over 2x BOD can get pretty nasty pretty quick for non-trolls. At body 6 that means neither of his armor ratings can exceed 12 before he starts nuking his AGI/REA.