I agree with Martin, Dave - census and other record images from Ancestry, FamilySearch, and other sites should not be copied to WikiTree because those images are copyrighted by the company that created them. The number or percentage of images isn't the issue - they're all under copyright. The census records are not under copyright because they were created by the U.S. Federal government, and therefore they are in the public domain. So you are correct, Dave, that Ancestry does not have an exclusive right to the census records. But they do have copyright of their images of those records. I have personally scanned and taken photographs of census pages from the microfilm at the National Archives - these images are mine, and I can post them.
Similarly with photos from Find A Grave, Pinterest, and other web sites - the copyright belongs to the photographer, so those should not be copied, even with attribution, unless we have the permission of the photographer.
Regarding census records, my personal preference is to cite the actual source I use, because not all images are created equal. This enables me to examine different images to determine a "mystery entry" when an image is not clear, or decide if I need to go to the original census microfilm.
As for ebooks, Wikipedia, and other materials that are available online, I generally either quote or paraphrase only a couple of sentences and provide a link to the online source for further information.
I do not know of an authoritative source that has provided an opinion for WikiTree use of material from copyrighted books. I believe that you have the right approach, Martin. Since most copyright allows small sections to be used in not-for-profit academic research, my personal opinion is that it would be acceptable to include a short quote with an in-line citation. I would not cite the source only in the Source section because that does not link the source directly to the material being quoted. Again, this is my personal opinion - I am certainly not the authoritative source you are looking for!