AV stacks with the specific rule of Custom Fit. No such rule exists for other modifiers, so one can make two assumptions.
There's no reason to make any assumptions. The rules don't state that you can stack other modifiers. If you do decide to allow people to stack other modifiers from armor, such as Social Limit modifiers, then you have to apply common sense to prevent abuse. Or the easiest, more foolproof solution is to not allow them to stack at all.
The rules also do not state that you can not stack these kind of modifiers.
As such, one of two assumptions is needed, as previously stated. You asked why I thought modifiers stack; this is my answer.
I think it only fair that someone who spends over 5k on a Berwik Suit and an Argentum Coat, for example, get the benefit of all modifiers from said items. That would include AV under the Custom Fit rules, social limit and concealability modifiers, and social dice pool bonuses, in this case.
One specific mention by Aaron is that special protection does not stack; i.e. Nonconductivity 4 on one garment and Nonconductivity 4 on another garment for a total of 8 is not a valid option. I consider this scenario, along with AV rules and the Custom Fit rules, to be specific rules that only apply in specific cases and not a blanket rule that applies to everything.
You may of course choose to apply the rules differently, but I believe there is enough ambiguity for both interpretations to be valid.
I think you're confusing what he said for what you thought. There are social bonuses for armour that aren't limit modifiers.
If you're referring to wireless bonuses, I have to ask the same question: why on earth would anyone assume that those would stack? Let's pretend we're using our wirelessly-enabled Berwick Suit to get a +1 social test dice pool bonus. How is that represented in the game world? The way I think it would make sense is for your suit to be able to project some tips on posture, give you advice on how to prevent wrinkling the suit, etc. I mean, it's not like the suit has little cameras and empathy software all over it. So why would having a bunch of items on you that give you tips like this make you have multiple bonuses to your social tests? It seems like the only thing that would matter is whatever you're wearing on the outside - so your designer underwear and socks might make you feel better about yourself, but they're not helping in social situations in which they aren't visible.
I have a somewhat different and much simpler outlook on this.
to my mind, the wireless social modifiers of high-fashion armor is just a wireless broadcast of the brand in question. Others recognize that you are wearing high fashion clothing and have a (generally) more favourable response towards you.
It's the equivalent of modern men and women wearing certain brand clothing (Nike, Kangool, Gucci, Armani, whatever shoe brands are in today, etc) as a form of status symbol.