Yeah can imagine it can be a long drawn out process selectivly breeding in/out the traits you want/dont want.
My power plant seems to have a hint of bubble gum about it now which is nice, thinking of crossing some with Blueberry and trying to shorten the plants natural height while trying to get a nice mix of the 2 flavours.
Never done anything like that before so it would make for some interesting fun.
I'd keep my true PP mother though and work from clones, dont wanna loose genetics totally.
As for ya stretch, dont imagine much you can do yet if you got the light well on em.
I use hps for veg too, i actually use duel spectrum bulbs for veg and flower.
Even at 6 weeks they were still just over a foot and really bushy and thats a sativa dom plant. Wasn't till i flipped 12/12 that all hell broke loose lol.
They are 6 weeks on 12/12 today and are an average height of 4.5ft, luckily they seem to have stopped or at least slowed way down.