My personal experience confirms part of what you have been told, but I wouldn't drop the daily hours all the way down to 12. Plants strech more with longer dark periods, so I would reccomend 18 hours for the indoor stage. Assuming the girls are going out into the great outdoors in early spring where they will be receiving something like 14 hours of sunlight a day I don't see any reason to drop your indoor light schedule below that amount before they make the trip outdoors.
Your sentence "the plants tend to grow and flower at the same time" isn't what you want. You want your plants to grow big and healthy in a totally vegetative state and then later in the season to sex and be ready for harvest before the first frost. Confused plants that think it is fall before it is are stunted and prone to hermaphroditism. If short healthy plants is what you desire buy a strain that is primarily indica, make sure it gets full sun, and if you still want it squater, top it once