For instance I have wrote my own fresh unique article which I will publish on mysite.com/sedancars (a picture gallery about sedan cars). May I use the same article on the same site on other pages? like mysite.com/sedancars/mercedes, mysite.com/sedancars/bmw, mysite.com/sedancars/audi etc. etc. The reason I want to do this is because my site basically don't need content, it's a simple photo gallery, but to work a bit on on-page SEO I want to put some content on it which will be limited. I have read a lot about duplicate content, but I thought in this case it's different since it's my own fresh unique content without copyrights and it will be used on just 1 site which is mine too. By the way; I will do some minimal changes on every page like the brand and model of a car. I will mention this in the title and in the body text.
I suggest you to tweak the content all the time for all the pages, in case you use the same small content. and if your content is lengthy, then i suggest you to go for unique content for all the pages. Because Google counts the internal pages content as a duplicate if its same, no matters how unique your content is... And try to keep your Meta tads like: Title and Description very unique for all the pages... Thanks.
Yes, but don't expect those pages to appear in the SERPs. Google try to filter out any duplicate content they find.
Particularly if you run your own online business like I do, you definitely want to have good quality content on your site to attract not only visitors and buyers, but to get the attention of the search engines that look for unique content.
Matt Cutts addresses some of your concerns here: http://www.mattcutts.com/blog/duplicate-content-question/ 2- I don't think you really need a lot of words on a page for it to have high PR. I don't think duplicate content would help you. (Though that's a guess.)