If I may just clarify it for you carrothead, being 99.9% sure I've got it right
The only armor that can be worn with other armor (or even just clothing) is reflec. The one exception is that reflec is not worn with combat armor (or battle dress), no doubt because they are already better and it would defeat aspects of those armors.
Reflec adds only to the laser defense (the number after the slash. So in the example of the Cloth armor and Reflec your AR would be 6/6
I think where Sigg is erring is taking the 'adds to AR' to plainly. It adds in the effect of adding AR 0/6 to the (for eg.) Cloth AR 6/0 for a combined AR of 6/6. I can't see where he's getting the idea otherwise.
Ablat is too bulky to be worn with another type of armor but does have some ballistic value.
IMTU it is the player's choice to layer the reflec above or below the other armor (unlike the rules that say it's only worn under). One note, if worn over anything you are a very shiny target
(can be a good thing if trying to be found or to avoid sunbleaching your favorite Hawaian shirt
If worn under the other armor then non-laser hits do not damgae it but if worn over the other armor each non-laser hit reduces it's AR by 1.
If the reflec is worn over the other armor then laser hits do not damage the other armor but if worn under then the other armor's AR is reduced by 1 for each hit from a laser.
The old house rule above basically grants an automatic critical hit against the armor only and only for ballistic armor hit by lasers and Reflec hit by ballistic rounds. Note too that Ablat suffers the same effect but only for each laser hit.
Combat Armor (and Battle Dress) should really be listed as a split AR of equal rating (eg. TL12 Combat Armor AR 7/7) because it is equally effective against both ballistic and laser weapons.
Reflec cannot be layered with itself to any advantage, though if using the MTU notes above, wearing Reflec both over and under other armor could provide some insurance if you run into both ballistic and laser weapon fire. In this case the AR vs lasers is still just 6 but if the outer Reflec layer is shot full of holes the inner one will still be intact, at least until the lasers burn off the ballistic armor and they open fire with ballistic weapons again
Something else to note, only Combat Armor and Battledress (of the modern armors) include head protection. Not important unless you allow called shots.
And as a curiosity I seem to recall that there was an article and house rule allowing the heavy archaic armor to grant it's full AR (the first number) against lasers if highly polished.
Also check the note for the AR for archaic armors on page 207 if you haven't already, not that your average Traveller is going to be found sporting say a suit of Chainmail but it could happen
Hope that helps a bit.