Suppose I meet a scholar at my university or some other institute, who is a famous person known for his or her work. and I ask questions related to the subject I am working on, and I get some answers or other useful information. Ethically, can I cite this meeting in my paper or not? (Provided I do tell the person that I am working on something related, etc.)
Strictly speaking, yes. The APA style, among others, does have instructions on how to cite private conversations (see https://owl.english.purdue.edu/owl/resource/560/11/ for an example), but in practice I've never seen it been used in psychology (or natural science) papers. This is probably because a personal conversation cannot be easily verified, which makes such citations less useful. One thing you can do is to track down the relevant papers and thank your colleague in the acknowledgement section.