Theres no reason why you can't start your plant out under your 250w HPS, theres not much difference in either HPS and MH during vegging. I use MH on week 2 of vegging and then HPS from then on. If i didnt have a MH lamp, i'd use the HPS. MH seem to keep the young seedling smaller and bushy, But the HPS put out more lumens, and thats what i want. It would be nice to see a side by side (HPS vs MH) veg grow journal. I really don't think there'd be much difference.
People say that it's better to start off under MH and then change to HPS during flowering but it's not like your "weed" plant isn't going to grow, lol..