imo, throwing them into flower won't stunt them, but will have the opposite effect on them really.. usually when i put plants into flower, the first 2 weeks or so of flowering the plants go through a stretch phase, where they will more then likely double in size, or more, depending on the strain, from the size that they were when you started flowering..
as far as when to start counting flowering, i have always been told to start the day you go to 12 / 12.. but saying this, my flowering time usually comes up about 2 weeks or so short of what the breeders say it should take for the plants to finish easily.. not sure if the breeders lie about the flowering times to make it seem as if they flower faster to sell more beans or if they tend to start counting days till they see pistils..
i have asked a few breeders when to start counting flowering and most of them will say at the flip, yet the times are always off, so i take it with a grain of salt..
also, the flowering times that breeders give tend to be a best circumstance kind of thing.. meaning that if you get the fastest flowering pheno out of the bunch, and everything goes to plan, then the times they give maybe correct, but if you get say a more sativa leaning pheno, the times could be way off by weeks and weeks..