For filling height in this tank, if you can, do it with plants. They will also help filter your water. I have the same tank, and my first scape used rocks and drift wood to build height. I redid it with plants and it looks far better and my fish don't hide nearly as much. Jungle Val is a good one. My vals, rotala, and cabomba all touch the top, and it looks great. My journal is HERE.
Also, I'd recommend skipping floating plants in this thank. The design will cause problems with that.
The white gravel will really wash out the CPDs, they don't seem to mind but they won't look as pretty as over a dark substrate.
I keep my CPDs with white clouds, have for about a year now and the white clouds really help the CPDs to get over their shyness. I've since added some gold ring danios and they blend in quite well and everyone seems to get along quite nicely.
I loved everything about my 12 gallon edge, except maintaining it