No, heat doesn't cause stretching. They will stretch if you do not provide adequate lighting or if the lights you have are too far away.
What kind of lights do you have again? If you are using CFL's, you can get them as close as an inch away. CFL's don't get hot so it's ok to get them really close.
As soon as the plants go into 12/12 they will start to make buds. Those are a bit pre mature for that though. I would definitely wait it out until they were a little more healthier then that.
But if you want to flower them now, give them 12 hours of daylight and 12 hours of night.