sativas love to stretch so be wary of growth spurts during flowering. i would say one plant could be manipulated to fill that space. yes you can clone a top it takes anywhere from a week to 14 days to sex a plant. all of this will depend on phenotype, like the old saying practice makes perfect. good luck

p.s. roots dont need to be huge clones that come out of cloning chamber will still need a humid environment until the roots adaptt to their new world. a week or two once you add veg time then you can flower and sex.

meded so you can mededicate to mededitate