Can an X-class safety capacitor be used in series with a load (i.e. where steady current flows through it)?
Can an equal value X-class safety capacitor with a 250 or 275WVAC
  rating be substituted instead


An X class safety capacitor rated for your incoming AC supply (not stated in the question), can be placed across live and neutral without fear of it shorting the AC out (if it fails) and thus causing a fire: -



Putting it in series with a bridge rectifier and load can only reduce the voltage the X capacitor sees so clearly it is under less stress.