Good Mornings are a very effective exercise. dont know if i would reccommend doing heavy squatting, and then doing heavy GM's. I would pick one or the other to do heavy.
i would highly reccommend you learn the form for doing good mornings before you start going heavy with them. like bignate said, there are many variations. if i was you, i would start with seated GM's, get comfortable with doing them that way, and then start doing the standing. if you have a power rack, do them in there. set the safety bars in the rack, so if you get in trouble, you can bail yourself out by setting the bar down on the safety bars.