Can full texts of research papers added in RG be cited ? Does such citations be considered in other media or pointers, such google scholar or h-index ?
If unaccepted by a journal, it should be cited as "non-refereed paper", or "work in progress" (or if it has been rejected, labelled as 'proudly rejected by Nature'!)
In my opinion, it should be considered as a pre-print and can be cited. It is then clear that it is not peer-reviewed and that the quality is author's responsibility. Though the paper might have been rejected for a journal, that is no clear indication of low quality. I have seen a number of papers rejected that I consider to be a valuable contribution. It may then be particularly important to get it visible since you don't know who has seen it and the idea may be copied. Possibly the arXiv is the best medium.