I got an ounce from a 1' x 1' x 7" cpu case and that shit was a bitch (1 plant). If you're planning on sticking 6 in there and thinking that you're going to be efficient, you have another thing coming. I'm all for breaking records and breaking the mold, but reality is reality...
I'm not saying it can't be done, I'm simply stating that you're better off growing one single plant correctly, getting an ounce from it and repeating the process. I've seen grows on ICMag, THC Farmer, High Times, etc. with people thinking the more they could fit in a tiny box, the more efficient they will be and that's not the case. I've seen people grow 3 plants in a 1ft square box only to yield half an ounce and I've also seen a person grow 6 plants in a 3' x 3' x 1' box and come up with an ounce and a half.
It's not because they weren't good growers, it's because no matter what you do you're going to be limited to your space, the amount of light that can penetrate every square foot of that space, and the size of the pot you plan on putting these girls in.
How long do you plan on vegging these 6 plants? What size container are you putting them in?