Are you set on keeping all the pothos? Succulents are something that usually do very well in vertical planters. They use less water and require less maintenance than a lot of things. They do need soil that is very well draining. Ferns also seem to work well in the living walls as long as they are smaller and have smaller root systems. One key is to use plants that have similar requirements for their culture.
Plant grow-lights are something to consider as well if you want to provide brighter growing conditions.
