MH bulbs are meant for vegging, they put off more of the blue spectrum which plants need the most during the growth stage. Vegging with HPS will work, though you may not see the same results as you would a MH bulb.
If you mix both MH and HPS I think it would be fine, a lot of people mix bulbs, I mostly see this with CFL's though, 2700k and 6500k bulbs mixed so the plant gets a full spectrum of light.
You can also buy an HPS bulb which is dual spectrum meaning it has both blue and red spectrum, perfect mix for both vegging and flowering, these are typically more expensive though.
I have mine under a 600 watt metal halide which I will then switch to the HPS during flower.
What lights do you have at your disposal?