Hi, I have two questions.

A: .Let us pretend that I have a piece of metal that is rusted and I want to buy a over-the-counter rust remover, how would ask for that to a sales person? Let us say that I can't remember any brand names for products that will remove rust in the metal piece. In my view, the 'rust remover' seems to be fine as a compound noun.

1. Hi, can you tell me where I can find a rust remover?

2. Hi, can you tell me where I can find a "rust" remover?

3. Hi, can you tell me where I can find a rust-remover?

Maybe I am not too sure on how to use a hypen and parentheses in phrasal situations like the ones above.

B: Sorry to ask one more question but when we have two candy bars, can we say like this (Note that the names of candy bars are made up for this 'question' purpose):

(Two names of candy bars are "Zo Zo" and "Xo Xo".)

The Zo Zo tastes much better than the Xo Xo.

Thank you.
A-- First, there's no point in presenting differently punctuated sentences-- speaking to the clerk nullifies any differences in your 1, 2 and 3. If you are writing a dialogue, use this:

1. Hi. Can you tell me where I can find a rust remover?

B:-- (The) Zo Zo tastes much better than (the) Xo Xo.
Students: We have free audio pronunciation exercises.
Thank you, Mr. M.

In the case of 'rust remover', it seems proper to write as such but can you give me some example two nouns linked/connected with a hyphen would be appropriate? What criteria, if there are any that can be laid out, are there for the purpose of making what I think is such a subtle choice properly?

a pencil sharpner -- OK, sounds correct and a typical dictionary would have it in its entries.

Let us say I invented a pencil that can also be used as an eraser.

a pencil eraser -- Now, it would seem awkward to write without a hyphen. Also, I think a typical dictionary wouldn't have it and it seems better to write "a pencil-eraser" with a hyphen.

What should I look for in making a correct decision in regard to this kind of things?

Sorry but is it correct to say, "A typical dictionary wouldn't have it" and not "It would not be in a typical dictionary"? When I think of the word "have", I have a strong image (or preconceived idea) of someone having possession of something and I am not too sure of the appropriateness of the expression "A typical dictionary would have it."
I haven't any examples to offer, just these guidelines: there should be no need to hyphenate when it stands as a compound noun-- only when it becomes adjectival: I need a pencil eraser. I need a pencil-eraser replacement.

Yes-- 'a dictionary has many definitions' is OK, though I would probably use 'contain' or 'include'.