Well, you have a 3'x6' area... That would be great for two MH lights, and for flowering.... that is, if you don't want to split it with vegging. Do you have another smaller area for your vegging?
If not, then I would consider placing it ABOVE the flowering area if there is a possibility of getting access for them from an outside wall. I would then place a shelf at the place you're gonna divide it..... say a height of 4' from the floor. That would give you a total of 4 ft height for flowering and 3' for vegging. Use the florous for vegging, the MH for flowering.

With a 4' ceiling you would not want them to get too tall, so you might want to flower once they are about 6-8" tall. Depending on how much room you need or want to get in there, I would say about 20 girls in flowering. The only problem I haven't solved is a space for your mother(s) if you need it.

/keylime