I know this won't help - but stick with photoperiod plants - less problems with genetics, bigger yields and less power consumption can be had easily enough.
Giving them 12/12 for a week may trigger the Blue Dream into flowering, but will not be ideal for your other autos.
Leaving them at 18/6 and the Blue Dream may not start flowering.
If after a week at 12/12 and the Blue Dream start flowering, going back to 18/6 may send them back into vegetative growth.
Leaving the whole lot at 12/12 to keep the Blue Dream flowering will mean a diminished yield form your other 4 autos.
What a dilemma!
If you don't have/want/can't afford another tent, lights etc to look after the Blue Dream plants, or have a buddy that could take them (problems, I know!), I really can't find a solution to your situation. Might be best to sacrifice the 2 Blue Dream plants for the 4 autos. If it is possible where you live, maybe putting the Blue Dream outdoors until the autos are finished, (if the climate is ok), could be an option, before bringing the Blue Dream back inside to finish?
This is what I mean about sticking with photoperiod plants, better genetics and fewer problems. As others have stated, this auto flowering problem seems to happen quite frequently, judging by the amount of questions on this very same matter. And this genetic "variability" caused me, after experimenting with 30+ autos, to decide that auto flowers were not for me or my situation. I encountered more genetic freaks over those 30 odd autos than I have seen in 30+ years of growing normal photoperiod plants. Others may love them, but for me, it is auto flowers no more!
I don't envy your situation, you really are "stuck between a rock and a hard place". Whatever solution you decide upon, I wish you the best of luck!
Hope this helps,......
Organoman.