Two birds One Stoned

Hey guys, im not a newbie but im also not a novice so id like some imput. Heres my small dilema. I have a 12x 20 room with one light, a 600W HPS. I currently have 4 ladies in there about 1.25 months into veg. There some random good finds I had collected over time so nothing super duper just really good. I also just got a dozen of Dark Night from a west coast friend. Im going to start the Dark Night in about a week after I germ them. The 1.25 month old ones just showed sex so I was originaly going to change over to the 12/12. Currently on 18/6. But i also want to start the Dark Night I got. Whats going to happen if I keep the older ones on veg 18/6 untill the Darknight is ready to switch over to the 12/12. Now obviously thats just going to veg out the originals a bit more right?Iis there a down fall or upside to doing this?

Also has anyone ever had the Dark Night strain before? ANY special insight to that as well.
Father is a Jack Herer and the Mom is Train wreck. Sativa/Indica Hybrid

Thanks again for your opinions and insight