To me the black goo was just a catalyst that formed DNA.

If you noticed, when they walked into the shrine/vault, they had worms from outside falling off their shoes? this then mixed with the black goo and created the worm/snake like creature.

When it entered Fifield, it only mutated him and his DNA alone. But when Shaw was impregnated, the results were slightly different as she actually birthed a mutation with part of her DNA.
Again, when this then impregnated the space jockey (I believe this ability was a result of being copied from Shaw's reproductive organs), the result was a humanoid lifeform (which to me was a really lame attempt at keeping the fan boys happy and I would have rather he left it out)

As for the other dead space jockeys, if you notice, their chests were not burst open but their skulls, the same as what happened when they tried to re-animate the head.
This must just be a side effect of mixing the black goo with space jockies DNA.

Personally, I think he should have kept it as the original planet from Alien and the ending should have had the space jockey make his way back to the space ship and leave the rest to our imagination.