What should I plant in our retaining wall?

We have a retaining wall at the back of our block made of boulders. I wanted to decorate it a bit by filling the crevices and gaps with plants. What plants should I use? I was considering some Rheo, would this work? We were also concerned that when it rains the soil/plants may get washed away as the water flows down, what do you think? Any help would be appreciated.