My question is related to this forum topic: http://turkishclass.com/forumTitle_40812 but anyway I think it will be better to ask it in this section.

Here is a quote from that topic:

I guess these words are best translated as "eyebrows / eyes", not "eyebrow / eye", in this context - in Polish, and I think that in English too, it is more natural to use plural form here. However, in this quote they are written in singular form. So I have few questions here:

1. Would it sound natural in Turkish if plural form ("kara kaþlarýn, kara gözlerin") was used instead ? Which one sounds "better" - singular or plural ?

Singular sound better but both are possible

2. Is it also common in everyday-Turkish-language to use singular form when in English one would rather use plural form - I don´t mean "number - noun" constructions here, like "5 boxes / 5 kütü", I´m aware about this, but I´m curious if in Turkish you can use singular form of noun meaning generally "more than one" thing ?

I think you can. I need to think about examples but for exampe you can call the soldiers as "asker" with plural meaning. Also during training in army they use singular second person when they are talking to the soldiers.

Nasýlsýn asker? -- How are you soldiers?

I ask these questions, because I noticed similar thing when I heard another turkish song, namely "Penceresi yola karþý". There are words "gelen geçen atýr taþý", which were translated this way: "passersby throw (little) stones"  (not "(one) passerby", and not "(one) stone"); and I wondered if this translation is really correct or maybe it was really meant "one passerby", not "many passersby" (and maybe he/she throws only one stone) ?

That should translate as

Anybody who passes by (meaning the passers-by in general)

You can use plural form when you are talking about some people specifically

Gelen geçenler oldu -- There were some passers-by.

Still if you omit the plural suffix the meaning doesn´t seem to change much. In that case singular form is preferred (no need the make it longer when you can say it in a shorter way)

Gelen geçen oldu -- there were some passers-by