You'll get tighter internodes stronger branches and less preflower stretch by increasing the dark period a few hours during veg. It changes the form of the plant when it gets a healthy dose of darkness. 18 hours light is on the extreme side. It's going to be a real shock to suddenly have a lot less hours of light they have adapted to in their life so far if going straight to 12-12. They are going to freak out and stretch growing stem in response for a few weeks. May be some help to glide, reducing light 30-45 minutes per week until they start flowering. Probably about 13 hours light will start some of them flowering. The damage is already done. From here out it's just damage control. You can manage.