For narrow spaces I like the Nandina domestica or Heavenly Bamboo a shrub family that is not related to the spreading bamboo despite the name. There are also tall and narrow yews and Ilex 'Sky Pencil' that behave nicely for evergreen color. The Nandina come in dwarf sizes at one foot and taller versions at about 5 feet with different foliage colors. You can also look at low growing heucheras and hardy geraniums to block out weeds around the base of the shrubs. Bulbs for spring color and hardy asters for fall color will add to seasonal blooms. Perhaps use some of those rocks to make a few rocky pools with some dwarf conifers around them. Have fun mixing textures and colors and it will be lovely. Keep growing, Marianne Binetti