If it gets hot, get the door open. You can't leave it open during the dark periods though. Well, you can, but you wouldn't want to.

If you can keep your temps firmly under control, and never hit 80F, I'd personally run 24/0 for veg - the plants grow faster with continuous light. Does that take more electricity? Sure it does...per day. But then you don't need to run it as MANY days to get the plants to the same size. But some people swear by 18/6. If you need a 'cooling down period' because it gets too hot, then 18/6 is great. If you don't, why slow down growth? I've heard some people say you get more females with 18/6, some books say that too, and some do not. I go by my own experience, and I haven't seen a male since April or May, none at all in my last 3 crops (which sucks because I was hoping to breed a Durban Poison with some Blue Mystics, but all I have are females), and I always run 24/0 (but I also have my temps totally under control, and high temps can cause more males, or worse, hermies).

If you veg under a 400+W *HPS*, I'd recommend getting some 6500K CFLs and use them along with it. That will prevent the stretching you see sometimes when vegging with a warm (light spectrum) HPS light.