Just due to height restrictions, I'd go with the 6"......if the vent fans are mounted outside the cabinet, you can get two more, of each size in there......... Whatever size pot and # of plants you choose......I'd advise getting enough growth to fill the space and pruning the excess to prevent crowding. With the 6", that's likely to be single cola...with the 9" maybe 2 tops per plant, with the 7"..maybe alternate 1 and 2 top plants.......... All of mine are in 4" rockwool set inside 6" pots for spacing, so I'm making the assumption that 6" soil will be enough root space for you as well......but I can't swear to it.
If you go with a Sativa....you will likely want to go 12/12 right out the gate. If you go with an Indica, you can veg to 8-12" or between 1/3-1/2 of the space from top of pot to the lights(start at 8" and adjust next go round)....an earlier harvest can give more of a head type high.....but I think different than the sativa head high. I can harvest at 6 weeks, but it's a very "Skitzo" buzz for me and I'm not that fond of it...I smoke to beat insomnia and that vibe just doesn't let me sleep....but some folks like it as they stay active and feel very "peppy"....
If you don't get suggestions here, you can go through some different strain/grow posts to find someone with similar conditions and stains you'd be intersted in trying. I have 3 1/2 feet from top of cube to bottom of light, but the HIDs need about a foot gap to not burn the tops, so when my batches go in to the bloom area, they are in that 8-12" range and finish right on the fringe of the burn zone......I can't honestly tell you my strain as it's from a clone my buddy gave me.
Best of luck..................