It would be really nice to not need a citation manager and just let Scrivener do the work with footnotes, end notes, and meta data. A good way to do this would be to create a “Special” Document for your Back Matter (or wherever) section, and then integrate a simple, multi-styled “citation generator” (Probably right next to the Name Generator), which would not only generate a citation for you, but would place either an end-note or a footnote mark [and the properly formatted. end-note or footnote] and place the citation, properly numbered, into the Citations Document, as well as give you an option to input Bibliographic data.
This feature has been requested before. We see citation management and generation as outside our expertise, and best left to one of the excellent applications already in the space.
In particular, given the vast differences in citation requirements across disciplines, and even between journals in the same discipline, there’s really no such thing as a “simple” multi-styled citation generator.
Better to use a purpose built Citation manger. For the Mac there is BookEnds, it works seamlessly with Scrivener.
There is a potential technical route that would allow Scrivener to “defer” the complexity of dealing with citation styles by bundling a CSL engine, as suggested here:
It would still require a reference manager program to generate the bibliography database, and all Scrivener would do is find-and-replace citation keys with formatted text provided by the CSL engine at compile time.
Pandoc does allow you to generate full citations directly from a Scrivener compile however (it does have a full CSL engine), and provides a whole bunch of other benefits for academic users; see my scrivomatic workflow for reasons why.