Ugh, so many questions...
Why is there a land squid? Is the black goo originating from a land squid-like creature?
The black goo seems to mutate things. Notice that, when walking into the room, there were little worms in the soil
Why does the xeno-mural not show the land squid?
Because they were a recent, unintended, creation thanks to the black goo spilling on the ground
Why did the black goo turn the spacejockey into DNA but turned Holloway into a squid-man? They are both human, right?
The ammount? Or possibly the goo changed with age. Third possiblity is that they were completely different types of goo.
If these SJs are cultivating life, what are the chances of human apes prevailing? What about dinosaurs and sea creatures?
I'm not sure what you mean by this.
Why did the Hammerpede go inside guyhoodie's mouth?
Maybe that good has engineered instincts and traits for the resulting creatures it creates?