Thanks for the question. Using an organic mulch e.g. wood chips, compost, leaf mold or shredded bark has many advantages over rock. It will help hold in soil moisture, also insulates roots by moderating soil temperature. Rock gets hot, which is stressor on roots and dries out the soil. Using organic material as mulch mimics nature so it provides habitat and food for birds, bees and critters whereas rock will not. When creating a bird/bee/critter friendly landscape, I strongly recommend choosing organic materials over rock. It will breakdown over time and 'feed' the soil, which benefits the plants and does require adding more material every year or two. Adding a light layer of mulch tidies up the beds and gives a finished look. Sounds like a fun project!

Paul -Master Gardener volunteer

Good question, thanks for asking! I would not put any plastic or fabric under the mulch so that the mulch can breakdown and 'feed' the soil. A layer of 2-3 inches of mulch will suppress most weeds. There may be a few that germinate that can be pulled/removed quite easily. 

Enjoy! Paul