Does the rule require companies to tag cover pages for ALL 8-K, 10-K, and 10-Q filings from operating companies? For all 20F and 40F cover pages that are annual reports? Or just those with financials in them?

Cover page tagging applies to all Forms 10-K, 8-K, 10-Q, 20-F and 40-F (for the last two forms, only if they are being used for annual reports and not registration statements).  The only exception is for filers who are not subject to Inline XBRL (i.e. ABS filers). Specifically, once a filer becomes subject to submitting any of its filings in Inline XBRL, it must begin tagging the cover page of its 10-Ks, 8-Ks, 10-Qs, 20-Fs and 40-Fs.

Is tagging of the 6-K cover page required?

No, it is not required.

Will there be new elements added to a taxonomy to tag the new data points on the cover page? Will that taxonomy be available in July/August 2019 when the cover page tagging starts?

Yes.  Currently, a June 2019 deployment of the revised DEI is expected.

The SEC mentioned Exhibit 104 as the Inline XBRL document to contain the inline cover page for an 8-K.  Is that correct? If so, would they expect all 10-Ks and 10-Qs to have Exhibit 104 containing the cover page for those filings?