Combining Microsoft options + custom Content Types in document library

I think by default it shows document templates associated with a content type. When you add a custom content type it'll remove the default options and will show templates associated with the custom content type. So it's not possible to show both of them together i think. One way to get around this issue is to add a document set template as content type along with your custom content type. In that way document set will have all the existing Microsoft template options and other custom choices would be available too.