I think you would be better off just sexing them out in half gallon to one gallon containers rather than start them 12/12 from seed.
Depending on strain and lineage you are sexing and then reverting back to veg, you can stumble upon many problems. This is not always the case but I will lay out two different scenarios, this is based on personal experience.
Scenario #1...
You start the seeds in small nursery trays (in my opinion and exp. the smaller area and volume of soil the seed is germinated in and where it spends it's first week to week and a half of growth, the quicker the seedling will grow) so think like this -->>> A seed started and spending it's first week to week and a half in a small nursery container will have faster growth rates than one started in larger container with a larger volume of soil. I believe the reason for this is because with the smaller area of soil the roots stretch out to the end of the soil and reach the "air layer" between the soil and the side of the container it is in, in turn giving it more oxygen to the roots than if it were in a large container and taking much longer to reach that "air layer". Starting seeds in a nursery flat allows more frequent waterings and less of a chance of over watering keeping the plant from stalling as often and allowing a faster transplant and in my experience each transplant always gives growth explosions.
Once you transplant from your germination tray or "solo cup" in your situation, after a week and a half transplant to a half gallon container. Within 30 days or a little longer in some cases you will see the plant has shown sex.
Most often a plant will sex quicker in a smaller container than it will a larger, just the same as flowering a plant in a smaller container will allow the plant to flower and finish a few days earlier than a plant that has room for root growth. I have left many mother plants in small containers and regardless of the light schedule they are under wether it be 18/6 or 24/0 cycle they will start flowering when they become overly root bound.
Sorry for going so deep into this and if I rambled too long about anything unnecessary. But point Is, start the regular seeds in a small container, after a week or week and half transplant to half gallon to one gallon container (depending on how many you have or if you have room to allow it), and do your best to not over water. This will allow you the quickest growth rates and the quicker they grow the sooner you will see sex on display.
Scenario #2...
You start the seeds 12/12 from seed and they still show sex in almost the same amount of time as scenario #1. If you get lucky you may see sex a week sooner. But here is the catch on that... And keep close in mind I have TONS of experience with this.
Sometimes the plants sexed from 12/12 from seed have went too deeply into the flowering process and built up so much flowering hormone that it can become a bitch to revert back to veg. I have seen them bounce back in little as one week and then I have seen them take up to over a month even under 24/0 veg and hefty nitrogen feedings introduced as well. A lot of times I have even seen this revert cause mutations in growth during the transition which is normal to some extent but at times I have seen them not come out of it and cause leafy, stringy bud growth patterns. This is not always the case but it is possible. I have done many reveges from sexing to revegging keepers that I had not got previous cuts from to create a mother.
However, sometimes I intentionally force a known mature female plant to flower after sexual maturity is naturally determined, then once she begins to flower I flip back to 24/0 hours to create a process which is known as "monster cropping" and this turns the plant into an insanely branchy bush that has its advantages if trained and pruned correctly, especially outdoors. But there is a "sweet spot" so to speak when you need to flip it from 12/12 back to 24 hour veg and if you miss that sweet spot you could have a long process in reverting on your hands that is definitely not for the impatient.
This is just my opinion, but in your case, I would just veg the plant for a week under 24/0, second week under 18/6 and then drop to 16/8 for the third and fourth weeks then once you see the exposure of genitals, I would immediately go back to 18/6 for a week then to 24/0 after a week of 18/6, giving it a heavier nitrogen feeding and a good dose of root growth stimulator "Real Growers ReCharge works excellent for this" once you start increasing the light cycle again.
Hope this helps, and hopefully the lengthy explanation doesn't bore you!
