No way to tell until flowering begins, if it starts at 3-5 weeks of age, and without going to 12/12, they are autos.
18/6 is the most effective light/dark ratio for autos and photoperiod plants in vegetative growth.
No, going from 16/8 to 18/6 WILL NOT cause any hermaphroditism.
Bag seeds always worry me though, most bag seeds are the result of hermies, meaning the seeds will have the genetics for being hermies too.
Hope this helps, Organoman.