Yes, you can clone from clones... that's basically what most of us do.
You don't need to flush them until the end. There's no nute burn... no need to flush. I use full veg nutes for the first 2 weeks of flower.. then I switch to transition to flower for a week and then bloom nutes for the next 3 weeks and flush for the last week. Last 2 days (before I chop) I throw the plants in a different room that keeps them in the dark for 48 hours before I chop.
Light degrades THC. So every night cycle the plant is conditioned to produce THC to act as a natural defense to light. Plants adapt quickly to environmental changes, but usually it takes 3 days for a plant to show any changes to it's environment. If you over nute.. you will start to see burn at about day 3 for example. So if you throw the plant in the dark.. it has all it's thc stored - ready to take on the day that doesn't come.. the plant will just produce thc preparing for the next morning... so basically you up the potency of your pot. Some people depute it.. but after trying both ways, I know the same plant (clones of the same plant) grown in the same room will knock you on your ass if I put it in the dark.. but may just get you high if I leave it in the room until chop.
You can manipulate your plants all through flower.. but I wouldn't recommend doing it past week 4 unless you run into a situation where the main cola is growing too close to the light and is burning. (have had to do that a few times).
You CAN flower too early.. but if you are showing pre-flowers you're good to go. The down side to vegging too long is our plant will outgrow your room. Bigger plants do produce more bud, but you have to try and keep it within the range where your light is effective or the bottom of your plant will just be fluffy popcorn bud, not worth the time to trim it. I'd say you are good to go anytime you want.
If you pinch the plant at the growing tips you will find the growth up stalls and they'll start to bush out. I'd try that on the shorter plants, and you could top the bigger plants.
Spacing is pretty important too. You can't just cram plants under a light and expect them to all grow like monsters. They need enough room to grow, and not have to compete for light or they will stretch to beat the band in flower- that effects yield, as the plant puts too much energy into growing to compete for light, when if it had enough light that was close enough, it would put that energy into producing bigger buds.
All shit you learn pretty quick as a grower. I totally recommend trying as many different things for yourself, you will find that you will do some things that work well for you that don't work at all for me.
also check out youtube. Honestly.. if you go there and search "marijuana cloning".. you will have a million videos that will walk you through how to do it... just watch until you feel comfortable trying it for your first time.. once you do it once.. you'll never worry about it again.. it's pretty easy.
Movie was good... seen it before.
Have to work tonight, but if you have any questions, pm me if you want.