Older growth is actually what your plant will be relying on for energy, especially during the later stages of flowering, so try not to prune any more than is absolutely necessary. A better alternative, in my opinion, is LST. You're a bit too far into your grow to make a good 90 degree angle bend to the main stem, but you can easily tie strings to keep the bud sites separated and in direct light.

Sometimes preflowers won't show for quite a while, where others strains will begin to show as soon as they're mature enough to do so. If you want to force sex, you may think about trying the cloning method I mentioned earlier. If you do this, I'd recommend taking two cuttings from each plant... that way if one dies, you'll still have a backup.

Or, you could just go ahead and flower them. These plants can easily triple in size during flowering, so if space is going to be an issue, that's something you'll want to keep in mind.