When shooting on a black background, a back light can be nice, to give you subjects some separation. Your subjects have light hair and you have a high light, so you don't really have spots that are blending into the background...but I still think a hair or accent light would help to make them 'pop' off the background.
I think the first and last would have benefited from a different pose.
Tilt and turn the head towards the front shoulder. Giving them an S curve head tilt and a 2/3 head turn. I think there would be more connection with the camera with that. Its also a more feminine pose.