Hey amma,
I had some pronouns in there, too - "it" for all of them!
I probably could have clarified further - for those "uncountable" nouns I think the easiest way to determine what to use is just to check is (singular) vs. are (plural) rather than trying to think about whether you can count it. And once you've determined singular vs. plural, the pronouns will have to fall in line, too.
So you'd say that "Pollution
is a big problem in Southern California.
It has been a source of controversy in recent political elections." "Pollution", a singular (uncountable) noun will never take the pronoun "they" or "their" - it's singular and requires "it".
So with that whole countable/uncountable thing, just check to see if the word is singular or plural and the regular subject-verb and pronoun rules apply.
Certain noun phrases (some of...; none of...) are trickier because the singular/plural distinction comes down to the actual noun after it (some of the pie IS; some of the peanuts ARE). Is that what you're asking about with "collective nouns"? Those tend to be the awkward ones but they're rarely (if ever) tested on the GMAT.