In nature multiple species typically do not live in the same niche in the same location. If they do cross paths, they will compete since they are filling the same role in their ecosystem and compete - eventually one will succeed while the others will cease. Welcome to evolution.
The same happens in your tank. Shellies relate to the substrate and almost all typically utilize shells in some fashion, so they will become territorial over them. You will have a small war if you put them all together. That's not to say in a large enough tank a pair of occelatus won't stick to the left side while the N. multifasicatus will move their shells to the right, but typically it's not a good idea to mix them.
I would highly suggest different tanks if possible. Didn't you make a rack of 20Ls? Those are a perfect size for almost every shellie! If you've really only got one tank, you can try building a divider to split it. It may work, may not work, but typically it's just not ideal.
