I am not a pro at this but from what I know i'm giving you my opinion/thoughts.
No, HPS bulbs do need ballasts unless they are self-ballasted bulbs but I don't know if any HPS bulbs are self-ballasted and I don't know if you can actually buy them self ballasted.
That is for you to find out.
Vertically hanging it in the middle of your ladies...hmmmmmm.........
It depends how close your ladies are to this light because you gotta keep HID(HPS, MH, etc) bulbs quite a distance away from the plants because they generate LOTS of heat and so will burn your plants.
If you can find a self-ballasted HPS bulb or if you decide to buy the ballast along with the bulb then I suppose you could hang it in the middle of your ladies but you would have to make sure not to get a reflector otherwise it would only shine like to one half of them.
If you wanted to do it w/o a reflector, I don't know if you can actually use them without reflectors, then you'd have to hang it vertically in the middle, make a circle around your light with your ladies and have a few feet between the light and your ladies.
Hope I helped,
Meeth