Production group shots = church photo directory

I am doing the photography for a church photo directory.  The
setup will be simple; I have will have the main light off to one
side and a fill behind the camera one or two stops down.  I might
have a light on the background too, don't know; will play with
that prior to the shoot.  The only outstanding question is:  How
would one go about setting up a hair light to accommodate anywhere
from one person to eight persons in a picture?

One thought is to place the strobe behind and above the backdrop,
dead center with the camera and shooting down on the victims'
heads.  I would use a 7" reflector and barn doors to keep the
light out of the lens.  My two main concerns is the extra space
lost to the light stand being behind the backdrop and that the
angle would be too low and light would still get into the lens,
even with a Lee Lens Shade on the lens.

I do have a softbox, a Chimera 1560 PRO II Bank that is 14x56in
(35x140cm).  I like the fact that it is long, if I bought a boom,
I could hang that directly over the subjects heads and it would be
long enough to cover a decent size group.  I am going to be
working in a room with 9 foot ceilings, though, is that enough
room?

I can also place the light on the opposite side of the main, but
my concern is that the subject closest to the main and those in
the front will get a lot less if any light from the fill and that
light will spill onto the sides of the victims' heads.

So, where would you place the light and at what power, relative to
the main light?

Sam