So they can't let them grow to 12 inches with that type of setup
I'm not to sure on your space, But I know in DWC, the roots grow massive and they will tangle with each other sometimes they will swallow your airstone up to, the only way that would be good to use if "you're friends" go with an SOG setup which is just single cola plants if you have 6 seedlings or clones that you know are Female you can go straight to 12/12 in this method from seed or clone, or you can veg them out till their a desirable height 6-12 inches and than flip 12/12.
But if they are all from seed that you don't know the sex of, I would tell your friends to either flip to 12/12 now to see what sex, take clones of the females and just restart with female clones in those systems with atleast a 2 week veg for 12 female clones, you will get very decent yielding single cola plants, or you could even scrog in that !