Depending on the species of moss (java is the most common) and the tank conditions, it can grow very slowly, or like a weed. I used to have it covering about half of the substrate in a 130. It looked pretty cool. It won't attach to the tubing. The bichirs probably won't use the clear tubing either, since they can see through it easily.
If you want to cut back on the light, try some watersprite. It's a very easy to grow floating plant. It can grow very fast, but you just pull out however much you want and trade it or toss it in the compost.
If you decide to make rock cave, silicone the rocks together. Better safe than sorry.
As for the background, when I had my store I decided to paint the backs of the tanks after they were set up. I just used plain latex paint that was black. I used a foam brush to put it on. Two coats and it was done, all with fish in the tanks.