MH during first few weeks of 12/12 to minimize stretch?

I've had a few soil grows with good results, and am on my first hydro with 6 plants. My new grow space is a little short (about 4 foot of useable plant growing height). So, I was thinking about staying with my 400 W MH bulb during the first 2-4 weeks of 12/12 flower induction to minimize stretch, and then switch over to HPS. Has anyone tried this before or heard of anyone else doing this?

In addition, I thought about clipping off the top nodes a few days before going into 12/12 to further minimize stretch of a single main cola.

I've attached a current pic - any comments about how soon I can go into 12/12? They are roughly 7 weeks from germination.

Thanks for your help and any other advice/suggestions you might have for optimizing my small 1st time hydro!

stretching is caused by the lights being too far away from the plant, MH and HPS only control the growth by light (photometric)