I read about something that I dont understand… can we use 12/12 lightperiod from the seeds without veg?
You can but you shouldn't. Without vegging your plant will be small, and thus your buds and yield will be small too. Give em a few weeks at least under a 18 to 24 hour per day light period.
You're better off using clones instead of seeds for SOG. Even then they'll benefit from a bit of vegging to get their roots established before the switch to 12/12.