need help

hi my name is greg im a first time grower and things are going a lot better than i planned my plants are only 3 weeks only but already taking a lot of room their over a foot tall an vert bushy eith branchs and leafs. i have a set up in my closet using a 250w hsp light, an extractor and fan i dont have a humidifer as of yet, a few have told me i should change the the cycle to 12and 12 to bring them into flowering seeming i dont want them to tall... basiclly is it ok to bring them into flowering now ?