so even the autoflowers need two types of lights?
It's not about need, just which is best.
I don't know a lot about autoflower plants, but know this, it is the light cycle that triggers the switch to flower.
You use 18/6 light/dark for veg (some use more light, up to 24 hours, but no less). To tell the plants it's flower time you switch to 12/12.
The plants use the metal halide/ hps different light spectrums more effectively during the veg/ flower cycles, but they don't
really matter.
You could use MH throughout the whole thing and you would get bud. You could use HPS the whole time and still get bud. Same with cfl's and floro's, you could use them the whole grow and get bud. If you want to get the dankest and biggest yeild for the electricity bill you are paying then an MH/ HPS combo would be best.
All that said, in my setup, I am using a 250w Metal Halide right next to a 400w hps for my flowering. I like the idea of both spectrums getting to the plants.
I know I'm not really answering your autoflower question, (i don't really know), but I'm pretty sure most of the same things apply.