Tank looks like it's going to be stunning once it's fills in
For the cave if you can't find rocks to make a suitable cave, what about covering the coconut in moss?
For foreground Ive tried most... DHG, lilaeopsis, styrogyne, dwarf chain swords, and I think styrogyne repens would look best in your tank. It takes a bit to fill in, but looks really nice once it does.
Your fish are gorgeous!! Is the first one a caucatoides or agassizi? What about the last one?