The red HPS lamp plus the photo-period change to 12/12 are what triggers the change to flower. So yes start hps with 12/12.
Yes a 1000 is big enough for 12 plants as long as they fit in a 6x6 foot square. A 1000 hps has a foot print of 8x8 feet.
it gets weak to the outside edge.

Remember your canapy can only b abut 2 feet deep. The lights don't penetrate any more than that.
KBS is how the big boy's play if ou want to google it. freaken trees.