is it possible to grown a plant from seedling to harvest just with a 600 watt hps

you can grow 6 plants under that and be fine. maybe even 10

 

Yes that is possible.

In fact that is what I do but my seedlings really liked it when I added CFL's (you can get the light much closer without heat problems).
So I use both now. I have a 600 hps and about 50 watts of cfl. Also it's best to use several small cfl's than one big one. It's cheaper and you can place them where needed.