I've used MH through both and HPS thru both. To me, the IDEAL is a mixed spectrum in both phases. But to answer you Q, yes you'll be fine. It's done all the time. In fact, if I had to use one or the other, I'd go with MH simply because it's easier on my eyes. You'll get just as much bud as you would have with HPS. Any difference would be measured in grams. As far as stretching goes, keep 'em close to the tops as normal. They will not stretch due to the MH alone. Some people say that the nugs will be fluffier and less dense, but that hasn't been MY experience. I've seen no noticeable difference.