Can you run A auto on a 12 - 12 Cycle?

I've never liked trying to juggle plants in different stages of growth in the same tent during flower. It's not only the amount of time for your light to consider but also distance from the plants. You may need to give the photo a booster seat in the beginning unless your auto is pretty small. Also it leads to gaps in your canopy later when you start chopping stuff...or it encourages you to add in something else and continuing the cycle. I like my flower tent to finish together. It makes cleanup and resetting to the next run a lot simpler, smoother and more efficient. I also mostly grow under a net, so partial harvesting is an extra pain.

12/12 is fine, as long as you add that extra light that is lost during those 6 extra dark hours, while still avoiding burning the plant with too much light. Like m0use mentioned, as long as the plants get the proper DLI it will be fine. I'm running constant 12/12, and move out small photos from the grow room if I want to extend their VEG time, parking them under a low 6,5W LED to keep them in VEG. A little bit of imagination and anything is possible hehe!

You can and get some decent results but the plant will never reach its full potential.

Sure, and if you do it correctly, absolutely no worry about losing anything with 12-hour operation. If you provide the same DLI, it will yield just as well as on any other schedule. Simple facts and not an opinion Hours of operation is not the full story. Light is a rate of umol/s of photons. We care about PAR or ePAR range of wavelengths. How much light given is rate of photons multiplied by hours of operation. To assess light in any other way is quantitatively wrong. To ignore one factor or the other is to stab blindly. Read the wiki on daily light integral to get the gist. Several studies show that the same DLI with varying operational hours results in the same yield. Fear not... As long as you give 150% more light over 12 hours vs 18 hours, it'll be the same result. 1:1 inversely proportional to hours of operation. Reference any DLI table to verify this simple fact.

yes. as long as the DLI is the same it is fine.

You can, but expect slower growth and reduced yields from the auto.

ah i see what you meant, you already have a photo flower cycle going? As long as you hit them with the proper DLI there will be no diff and no yield loss. You can lift/lower the autos themselves to dial in the DLI you need while keeping the photos dialed in.

You can might lose a little on the auto, ya could stagger the harvest just let your photo veg for a while grab a couple clones before you flip

absolutely, as long as you still hit the proper dli's it wont make a difference in any way. Obviously you have to run a much higher intensity that you would vs 18hr. Dont know why you would though, id much rather have the lights on longer at a lower intensity

No, they all recommend 18/6 or 20/4 hours. 12 hours will give you a low yield (For Auto Strains).

They flower on age no matter the light hours. I'd expect them to be 30% smaller due to 30% less light besides that they'll be the same.