The top of the slope might be too dry for hydrangeas since moisture would want to drain downhill. They might even want afternoon shade in your area. You might look into shrubs that are a more drought tolerant and interplant a few varieties that bloom at different times. Their roots might be just enough to hold your hillside together depending on how many you plant. You could mulch any bare spots with shredded leaves or shredded hardwood mulch. To me, ground covers almost always mean thugs or invasives.
