Possible to have document and item content type in same list or library?
It is not possible - document library allows only document related Content Types. In case of Custom List, you will find Document Set on a list of available content types, which may indicate that storing documents within custom list is possible, but when you try to add this content type, you will receive an error.
Only option (which may match your requirements) is to add Document Set content type to a document library and add required fields to it. This way you will have documents and item alike entries. But basically document sets are not meant for that.