It won't get any slower than it already is, so it should just speed up in the 18/6.
And you don't have to wait till it shows sex to flower. In fact, it won't show sex until you flower, after about 2 weeks of 12/12 normally. which you can do whenever you want. I would say to let it grow a lot more before going into flowering though. The more foliage and growth it has when you go to 12/12, the more bud you'll end up with at the end.
A rule of thumb is that the plant will double in size during flowering, so you might want to switch when it's half the size you want in the end.
Anymore questions/conserns? I might be able to help.