I used only 400w MH (all I had at the time) for my first 2 grows, veg and flower. Since then I have tried using HPS thru both phases, and a mixed spectrum both phases. The 2 spectrum thing is supposed to more accurately replicate the seasonal sun (thereby increasing yield and bud density) but the girls certainly will not die or do poorly if you use only one spectrum. In fact, IF there was a difference in yield using both lights as opposed to one or the other, it would only be measured in grams in a relatively good sized grow!!
Almost all of the little "tweaks" you read here are just that...and will have little or no visual/discernible effect on the general well being of your plants. Don't obsess over all this. Your plants aren't gonna die, and will do just fine under a basic light set up as opposed to a more refined system as long as you use HID or floro in one of the proper spectrum's. Now try an incandescent or Mercury lamp and you'll have troubles.
Give a plant Miracle Gro and it will do well (used properly of course), but we can tweak the nutrient regimen and have them do better. Same with lighting.
I remember the first grow, I stressed out bad, because everything I read on the forums said to use MH for veg, and HPS for flower. I have no idea why the mind "loses it" and ignores science when it comes to MJ, but I was convinced that my girls were gonna die as soon as they tried to flower under MH. LOL!