I think whether you make the streetlight effect soft or hard is more a matter of the mood of the scene - realistically it's hard-ish, roughly a 1'x2' source 20' in the air, so not pin-sharp but not soft.
But I've hung a soft box from a small condor over the scene for more of a soft toplight effect. I've also used hard sources. I've used hard sources with light diffusion frames, etc.
Since often you are balancing to low levels on the street, you can get away with diffusing a light or using smaller lights, etc. especially when shooting on a digital camera that can be shot at 800 ASA and higher.
What color gels I use again depends on the mood I want for the scene, and how much fleshtones will be under that light. For background lighting, or a backlight, it matters less, but you might not want some overpowering color on faces for a long dialogue scene.
So for sodium street lamps, I'll often just use 1/2 or Full CTS or CTO if faces are prominent.
If not, I might use Rosco's Urban Vapor. It's sort of an apricot color. They also make a gel that is closer to the real color of sodium, it's sort of brown-green, not pretty. See:
http://www.youtube.com/watch?v=o0ACnfdhPks
For metal halide / mercury vapor, that's a cyan color -- the issue here is whether you are putting the gel on a tungsten or an HMI light, because cyan is blue + green. Metal halide, which is more modern, is closer to the look of Cool White fluorescents, I'd probably use 1/4 CTO on an HMI plus 1/2 Plus Green. So on a tungsten, that would be 3/4 CTB and 1/2 Plus Green. Mercury vapor is a stronger cyan -- Rosco makes a Cyan gel series that works well on tungsten. Otherwise, I'd just find some cyan gel combination.
