I have a question about agreement between words that are ALWAYS plural. My understanding is that adjectives and nouns and pronouns have to agree in number and case? But what if I wanted to say …”This Elite (figurative) Night” эти сливки (ночь works or the noun has to agree with the always plural elite/cream as in ночи)?
If I understand your question right, you are making some assumptions about Russian which are not quite correct.
In Russian, there is no such thing as a noun adjunct. If you want one noun to modify another noun, like in "race track" or "sea view", you have to do something with the modifier to make it work this way.
Usually, you have two options:
- Make an adjective out of the modifier: гоночная трасса; морской вид, literally "racing track; marine view".
- Put the modifier into an oblique case: трасса для гонок, вид на море, literally "track for races; view on the sea".
Not all of these options work equally well for all the nouns; and the cases and their corresponding prepositions, if any, vary with the noun. You have to learn in separately for every word.
If you use the adjective, it does have to agree with the noun in gender, case and number.
If you use the noun in the oblique case, it does not.
In your example, "the elite night", if we treat the word "elite" as a noun adjunct, the only viable option would be ночь элиты, or, if you will, ночь сливок общества. The word сливки is in plural genitive and stays this way regardless of the case and number of the word ночь.
If we used another example, "the Christmas night", it would be рождественская ночь. Here, рождественский is an adjective and has to agree in gender, case and number with the word ночь.