How well would the finish match if I buy a PTR-91k and a seperate original collapsing stock from somehwere else? Assuming the stock is like new...

And which variations of this stock will NOT fit the PTR rifles? I'm pretty sure HK91, HK33, HK93, G3k all are compatible or identical, correct?

nope, 33/93 are very different, single pin models, G3=G3K=HK91=PTR and those are all two pin models

degree of finish match will depend on what generation, what year, etc.  I have an early G3 collapsable from the 60's that is a fair match  in terms of finish and the newer ones are a very poor match for my PTR wtr finish

If you want the whole finish to match perfectly, you will probably have to refinish the whole package together as a unit

Exactly what I needed to know, thank you very much.

All the HK collapsing stocks I've handled were sort of parkerized. Got one to match my PTR nicely using Rustoleum Satin lightly applied, then baked. If you do this, be careful to avoid applying too much on the inside or it will make it too tioght when collapsing it on the rifle (obvious, but I still made that mistake )