Real quick question: building a knowledge junkie Prime Runner NPC; y'know, the kind of guy who's a walking encyclopedia and need some advice regarding qualities:

Throwing in School Of Hard Knocks, Technical School Education, College Education, Linguist, Analytical Mind and Speed Reading, and then throwing in Learning Stimulus Nanoware rtg 3, Recall nanoware rtg 3, and a Mnemonic Enhancer rtg 2. (and Media Junkie and Compulsive: Knowledge Junkie...)

So, by my math, as soon as he puts any points in any knowledge skill excepting interests, he basically has it at rating 4 (1+knowledge quality + enhancer), and can raise it for minimal karma (house rule: Learning Stimulus reduces karma costs to a minimum of 1), and languages immediately jump to rating 5 (1 + linguist + enhancer). My question is, do these stack or not?

A modified skill cannot exceed the base skill rating x 1.5 SR4A P118 so you need a higher base skill for all the bonuses to apply.

mnemonic enhancer only gives a dice pool bonus; it does not actually raise knowledge or language skills.

One of my least favorite rules. Oh well.

one of the best rules. That's why people keep "forgetting" it.

Huh. Does that mean that knowledge and language skills have their augmented ratings cap at 9 as well? Or do they just cap at 6, meaning that you only need to have 4 ranks and a +2 modifier?

I assume they would cap at 9 just like active skills, although keep in mind that other than the qualities you mentioned earlier, there really any boosts to them (even aptitude and improved ability are only for active skills, although I would probably house rule to allow both to be used for knowledge skills, if a player was that gung-ho about a particular knowledge skill). Things like mnemonic enhancers would not be affected by the augmented rating at all, since they are dice pool modifiers, and thus, not subject to the cap. Same with some geneware treatments like PuSHeD.

It's hard to figure out how those qualities work out in play - okay, the linguist quality. The others are fairly easy - you buy them up to two, then they suddenly "jump" to 3. Linguist will do that, too, but when do you get the second point? I assume you have to buy ranks 4 and 5 normally, and then it "jumps" to 6.

Interestingly, this rule is stated in every context with skill enhancers (in the description in the rulebook), exept for the knowledge/language enhancing qualities. Maybe this rule is only meant for active skills.

It's always a mistake to read the tea leaves in a way that helps us. All of these probably should have been written as DP mods, and yes, they should have either said the R*1.5 rule was general, or specified. However, we work with what we've got. It's a reasonable assumption that Knowledges *aren't* uniquely uncapped without anyone ever even implying that.

Still, unless your GM is also giving away 'complementary' Knowledge dice to real skills, it's probably harmless enough.

Agreed. Typically the more I dislike the rule the more it keeps my inner munchkin in check.

Why do you think linguist works any different from other, it should work exactly same, so:
Base rating/rating with linguist
1/1
2/3
3/4
4/6
5/7
6/8

Apparently, as of SR4A, the modified rating is used to determine the cost of improving a skill. Something I would seriously consider house ruling to how you have it, just to make skill costs simpler.

I cant find this particular rule anywhere, so a quote would be appreciated.

Well there is this for skills:
and (less relevant to this discussion) there is something similar for attributes:

Only problem I see in this thread is the vast majority of those things are dice pool modifiers, not skill modifiers. Especially the qualities.

Look at Mnemonic enhancers, then look at Reflex recorders... the latter is a skill modifier, the former isn't.

About the only way to limit dice pool mods is to kick in things like some of the suggested house rules. EG: no dice pool can exceed 2x skill+attribute.

Yeah, the problem of a system that slightly relies on Skill Mods and largely relies on DP mods (and, just to be a pain, sometimes Threshold mods) is that it's not really balanced for any of them. Personally, I'd specify that X is incompatible with Y and be done with it.

Oh, how very nice of them to not include any mention of that to the actual chapter that is about improving characters in play