first, you can save some money and veg with CFL's rather than the MH.
second, waiting for the plant to mature is the right thing to do.
if headroom is an issue, grow out the seeds, take clones, flower clones, shitcan male clones and parents, save parents of the best producing fems and start taking cuttings that you can flower as soon as they are firmly rooted.
do that and you are almost guaranteed as many female plants as you need and they will all grow at the same rate (being genetically identical) plus, they should all have the same potency, all other things being equal