18/6 is optimum for all cannabis, autos or photoperiod plants in vegetative growth.
It will not harm or reduce yields compared to running your lights at 20/4 or 22/2 or the worst option of all, 24/0.
You have also set yourself up for other problems by having both autos and photoperiod plants in the same tent.
Unfortunately, your photos won't flower at 18/6 and your autos won't yield very much at 12/12.
Unless you have planted the photo seeds near the end of the autos flowering cycle, they will grow huge and overshadow your autos by the time the autos have finished flowering.
Flowering your photos earlier by going to 12/12, will mean your autos will not be getting enough light to grow and yield to their best ability.
It really is best to have all autos or all photoperiod plants if growing in a single tent.
Also, be aware, most "bag seeds" are the result of hermaphrodite plants, meaning they have a genetic dominance to be hermaphrodites themselves.
Hope this helps,...........
Organoman.