As I said, my reputation is hated by both the Legion and the Powder Gangers, yet when I encounter their groups, they don't attack me. Is that right or a bug of some sort? By the way, I am not wearing the uniforms of either group, just some regular old metal armor....

In my experience relations are bugged (or programmed in very irrational way). I had some odd behaviour with various groups too and many people have reported odd things as well.