you cant tell a sex untill ya flower so put them under 12/12 and by week 2 you should be able to tell if ya are wanting to reveg just change the light cycle back to 18/6 and it will revert back in to the veg cycle without any probs but ya cant reveg once the bud starts getting big i think this will stress out the plant and turn a feamle into a male so try and do it in the first couple of weeks
cheers hope this helps