Yes, you caan have your light 18/6, 16/8. Many people vary thier lighting paterns, but remind you that this lighting schedual is only for Vegatative growth. Depending on how long you veg for and when you want to start flowering, then you will want to change your schedual to 12/12. Remeber that whenever your starting to flower I personally recomend that you slowly take hours of daylight away. If your veging on a 18/6 schedual, I would take and hour or two of daylight away each day until they reach 12/12

Good Luck and Happy Growing. Keep us posted on how thier doing!