Plagiarism and credible sources
I'm not sure where you heard that you should only cite &quot;credible&quot; sources. Credibility is a judgement call, best avoided. You should always cite things that you have found and used, even if they were ill formed when found. The ideas are those of others that you have developed further. But the idea didn't originate with you, so you are open to charges of plagiarism if you don't cite. The likelihood of a formal charge may be slim, but other academics might doubt you if you cut corners.
That doesn't, however, mean that you have to accept the veracity of the things you cite. In fact, you can, and in some cases should, discuss the doubts you have or the ill state in which you found a resource that you use.
I'll note that in mathematics, we sometimes have to cite papers that contain serious errors so that we can correct those errors. But our work is still based on the work of the earlier mathematicians, even if we correct it.