Someone can experience these emotions, but not at the same time. To be at peace and full of rage would drive someone mad.
The Voss don't do this, they are naturally gifted with raw force Energies. Revan didnt have raw force, an example of raw force is Galen Marek or Kyp Durron who friggin exploded with energies without any control. There is a huge difference between those guys and Revan or Vader, and I sincerely hope you see that.
It's not possible. Even the most powerful Jedi can't draw upon both sides simultaneously. We have seen Luke say that there is no grey area, that you are Light or Dark. It is obvious that the most powerful being ever wouldn't try to be grey. To say that Revan can do what Luke cannot do is really... um... wrong.
As to the Voss. I just recently finished the Voss storyline and I have to agree with you. It is pretty obvious. The Voss don't have the same energy that Vader or Revan has.