The below is summarized and paraphrased from The Craft of Research: Second Edition, by Wayne C. Booth, Gregory G. Colomb, and Joseph M. Williams. This is currently among the reigning standards of research manuals across academic fields.
Sourcing and the reliableness of sources is a tremendously complex issue; there's a reason most universities have an entire class on writing the research paper. No policy page can possibly solve your sourcing problems for you. In the end, these are principles - actual guidance will come from the consensus of your fellow editors.