Nope Dietrich isn't a Neo-Nazi, Rozencreutz are something MUCH different, pre-dating Nazism, more akin to Da Vinci code style kind of secret society kind of thing. It's pretty clear in the manga that it's basically a backlash organisation against the Catholic church, which of course in later times was a Nazi thing even though Hitler and many Nazi's were originally and some even stayed catholic.

Na I think Gackt was following the angura idea of the uniforms (he liked wearing WWII Japanese uniforms in MM so I think it's more a throwback to that). Gackt was probably trying to be edgy and bring a bit of angura style into what he was doing since R+R was a very experimental album for him. Mars might of had different styles of music on it, but it wasn't musically experimental like Rebirth was. I think he was trying also distance himself from VK in the same stroke since angura style is kind of anti-VK even though there are costumes and makeup etc. Also Gackt has strong nationalist feelings so to a certain extent so the angura philosophy appealed too I think.

ps Angura Kei >>>>>> Visual Kei