Are the plants you want to clone from clones themselves or were they grown from seed? If they are from seed then you should wait until you know the sex (a week or two into flowering) before you take clones. You only want to clone females 
If they are clones from females then at 12" they should be fine to start cloning.
You never want to take clones from female plants unless you absolutely have to. Once plants are flowering it takes a long time for them to get back into veg mode.
This is just my opinion but heres what I would do.
Take clones from your 12" vegging plant. Put them under a fluorescent light outside of your grow room. They don't need much light as clones. A fluro will be just fine.
Your clones should root in 5-10 days. While you are waiting for your clones to root start what's left of your 12" into flower (12/12). By the time your clones are rooted your 12" should have shown sex or should soon.
If your 12" shows that it's a female throw the clones you took from it before you flowered it and put it in your grow room under 12/12. They will be small and this will be SOG style. Flower them out. *Don't put all of the clones into flower. Save one or two for mothers*
Because the 12" started sexing a week before you will get an advanced preview of what the rest of the ladies are going to do. While your ladies flower, grow out the one or two clones you kept in veg state and let them get big.
A few weeks before you are ready to harvest take clones from your mothers and get them rooted and let them get a little bit of vegging in. Once you pull your ladies from the flowering room for harvest replace them with your clones that have rooted for a week and vegged for 2 weeks and there you have it. A never ending cycle of goodness.
That's just what I would do
