Just some thoughts on the piping you have I use to do the same thing in the mid 90's with my reef tanks to eliminate dead spots (man I wish the current technology in power heads was available then) the big issue is having a big enough pump to make the flow worthwhile, all those bends really kill flow and most water pumps are high flow at the output but die out quickly when any pressure is applied. I just wanted to give you a heads up on that because I would always go bigger than you think you need.
Another though is the rocks if you want them to stay bare then the really smith river rocks are a good idea if you want plant to easily grow in them I would go with something that has some texture. You can also mix the rocks so you have a smooth river rock peaking out of a clump of Anubias or fern. The other issue with smooth rocks is they are hard to lock in place just ask anyone who uses them in their African ciclid tanks, they are a disaster waiting to happen as the fish dig
I would also stick with only African plants, if you are going with a Biotope then half the fun is collecting the plants and animals because it isn't like there is an all African section at the store so you have to do research, my Borneo tank has been a blast to add plants to and I consider it a huge win when I find a new plant from their. Saying this I would stay clear of the Java fern a it is more Asian.