Bit of a mixed bag of questions there but here goes:

XMarks just saves Firefox bookmarks and it doesn't matter what OS you
are using ( Windows or Linux ) as the plugin is a Firefox one.

If using Chromium to view GMail then there's nothing you need to do to
save setting / mail etc... everything is stored on GMail servers and
you can access from any browser on any platform ( Windows or Linux or
Mac ). Same with Yahoo Mail - it's web mail and nothing is stored on
the PC itself.

AFAIK - GMail account sync is for sharing contacts etc between GMail
accounts you have. If you only have one GMail address it's of little
use :-)

Does that help ?