You really made an investment in river rock! I hope it turns out to be the end of your pest problems. River rock typically holds not water but heat--meaning, while it does help to hold water in the soil, acting as a mulch, it also will help to moderate the temperature because the rocks hold heat. You might discover you can grow some borderline hardy plants with success in the rock-covered areas.
If there are any plants that are supposed to be heat-sensitive (don't like hot roots), don't try to grow those. (I can't think of any plants like that, but felt to mention it.) Otherwise, you can pretty much plant whatever you like. Research studies have actually proven that river rock mulch helps warm the soil more quickly in spring, resulting in larger plants faster in a landscape setting. Isn't that terrific to hear?!
Ornamental grasses are beautiful with river rock mulch, as is phormium and yucca (these are foliage-style plants). Many people like to plant Japanese-style gardens among river rock, using many foliage-type plants.
Azaleas will grow in river rock, and almost any other shrub you can think of. Trees do well, too. So you can basically put all your favorite plants into place in the river rock. Have a great time doing it!