MH does keep vegging plants shorter and tighter, and whoever mentioned raising the lights is correct that it'll probably aide stretching a bit if clones are what you're shooting for.

As for using HPS for veg, while I won't say that it's any 'better' than a MH or blue-spectrum fluoros, I did use nothing but HPS for quite a while before I ever discovered conversion bulbs and switchable ballasts. I never could allow the plants to grow vertically, or they would have damn near reached the ceiling, but I did get to do a lot of experimenting with LST methods due to the extra stretching, as well as topping/FIM techniques which I believe may have improved the eventual yield.

So, all in all, HPS in veg might not be ideal (depending on what you're looking to achieve), but it works in a pinch, and in this instance, would almost certainly have assisted in cloning at such an early stage.

Just my :twocents:, for what it's worth.