Yeh just as @cogitech says.
Feed with veg nutes for 5 weeks then onto the budding formula if thats what you are into.
Heavy Indicas can be quite small sometimes so I would recommend 50/50 indica/sativa strains for a bit more yeild.
12 12 is a great way to grow for lots of variety and its easy to just add more seeds whenever you have space available.
Keep smiling.
Thank you for your answers, iāve already a seedling of bubba kush cbd on 12/12, in a pc case (very crazy) and this is my first micro growing experience so i dont know anythingā¦and what about consulting old overgrow archive? Iām very courious to know if it Is it possible to restore old threads from a backup?
No, the old servers and threads are no more. There are plenty of knowledgeable people here though.
Ok, thxā¦there are server that have some copies of web sites, i 've tried google cache and archive.org but without success. Ok, thx again 
As before mentioned alot of strains come from equatorial areas that have 12/12 year round, thatās why some alot of the plants you see grown outdoors there are so short, depending on the strain this could be beneficial to allow it to flower when ready vs switching from 18 on down to 12, there is a reason we call it force flowering ,tricking the plant to flower based on a severe reduction in light. If anything that might cause stress if plant wasent genetically ready to flower ,forcing it to flower and survive due to a drastic environmental change. There are so many hybrids now they might not react well to it but most equatorial sativas I would expect to benefit from a more natural light cycle, old literature Iāve read says some will flower after only a few weeks and some may take a year definitely something I have always thought about trying for comparison. The four month flowering sativas could finish on time if they started flowering by end of July and prob still be the size of a standard indica type varieties
The cold of winter does slow them down some, even in my sub tropical environment, but as you say the Sativa type strains do get a lot bigger with 12/12 from seed. To be honest so the plant gets out the gate as fast as possible indoors I would still give them at least a couple of week of veg. Outdoors in winter obviously we donāt have that option 
Further north from here and closer to the equator itās possible to have 2 crops per year with the āwinterā crops only ending slightly smaller and less dense, but this seems to be the case only with sativa type strains. Where I am even in summer to get an indica strain to go full veg without just getting to 6 inches high and going into bud I need to plant as late as mid November and harvest the end of Feb, whereas Sativa can be put in mid September and not pull it till mid April.
Back in the day (old OG days) 12/12 from seed was fairly popular. I grew this way a time or two and found it to be fun. I had one tent that stayed at 12/12 and spouted them then into the tent they go. Now Autos have evolved to the point that 12/12 from seed is either just a lark or to get a peek at a strain before you pop 25 beans! Itās damn good fun though.
Then let us think about the question in a way that isolates and addresses your stated problems. Too much root growth for the required veg time from seed and seeds need a minimum time to become mature enough to flower.
My first thought is that as you are growing HPA you do not need a taproot, which is one of the main reasons to grow from a seed. Instead, you actually want the sort of lateral root growth you get from clones. Lots of fine roots spreading out almost horizontally from your stem.
Additionally, using clones, you can go straight to flower once rooted although 2 weeks veg gives greater benefits (depending on your grow chamber size). I run clones and I give them between 9 and 14 days veg and with between 8 and 9 weeks flower this gives me 1.3GPW with an average dry weight of flower per plant of about 180g (6 oz).
I have my mother plant in a bubbler with a tapwater top-up float valve in a side tank syphoning into the bubbler under a CFL bulb. I often leave it alone for up to two weeks and while it needs a lot of TLC after so much LITFA, it lives and I can take clones from it. I have recently started a new mother as the old one had too much old growth on it.
For your system, I would advise to run a mother plant and clone area and then you can fully control the veg time. You can judge when to flower them based on the size of your rootball, which is the best way to judge it in a highly tuned system IMO.
You can still sprout seeds but put them in the veg room and clone them, then you do not have to take the initial time into account while you wait for the plant to go through puberty. This removes the problem of root growth during the time between sprouting and maturity.