yeah it would help, when i did this my normal plants grew way to big and i had to flip them into flower, resulting in the auto to only get 12/12 and results drastically decreased. just make sure you keep at least 18+ hrs of light on in there.
So you are Scrogging 3 Photoperiod plants and 4 Autoflowers? w0w
What light and shedule are you currently running up to now?
Are you asking when you should use the HPS and induce flowering on the Photoperiod plants, i mean if so then i assume you will be flipping the schedule to 12/12 Lighting? Did you consider the consequences this is going to have on the Autos because they need more than 12/12 schedule for optimal results?
400 W mh lamp period is 18/6. autoflower's are flowering on mh lamp good ? or they need to hps lamp ? But photoperiod's wants more mh if i go 18/6 hps lamp for 2 weeks for autoflower's and then go 12/12 hps lamp for them . is that good ? i just want high thc lvl