Plastic would probably cause rocks to slide or soil to slide (less friction), and might slow the drainage process. I agree with Zoe:!
Sometimes we might be a bit slow to respond but this is a good place to start! Since you are in Colorado; cold hardy plants; consider visiting the Sempervivum Forum and the Sedum Forum ! Another good place to ask.
The gravel could be removed and re-used as topdressing if you like the size and color. You could also go around looking for large rocks or boulders to put in
If you need help with designing or that stuff you could probably do it here too.
