I'd guess a food source, mostly. But they have been capable of surviving in very harsh environments. Areas so hot it damages Samus from within the confines of her power suit, and equal extremes of cold temperatures. The terrain of SR-388 is their main habitat, and I think it's similar to Earth. But you'd get a more certain answer out of Cyner or Scenario.
I can't say if Oxygen or anything is required, but I wouldn't think so given what they appear to be. Basically just flying blobs of...stuff. Again, Scenario or Cyner could probably answer this better.
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All hail Scythe, King of the Sigs.
Gender: Male Location: 1/9.7'rd Horseman of the Apocalypse
Killing an X-parasite's host does nothing to the parasite itself, and infact, the parasite can make it get back up, or reform it, as was done to Ridley's corpse by an X parasite in the same game. The parasitic copies are perfect, reproducing all of an opponent's abilities and even their tech. In Fusion there were several SA-X's formed from pieces of Samus' armour which were equipped with her entire arsenal.
The Zerg, while numerous, I don't think have any weapons capable of stopping the X. Killing a host releases the parasite which can just infect you, as well. It could be called a battle of endless numbers, but the truth of the matter would be that it's not. The X never become fewer, only more the more Zerg are thrown at them, and the Zerg while numerous are finite. The X would get them all eventually. Taking over a space-faring form would probably mean the end of the Starcraft galaxy, it was implied in Fusion that if the X spread they would be unstoppable for even the Metroidverse.
This is all assuming the Zerg even get past the Metroids, energy draining parasites which can grow into some nasty things. I'm not sure how the Zerg would go about fighting the Metroids, they're... Quite durable, even in their most basic form, and usually require being frozen to be handled effectively, the larger they get the harder they are to kill.
I would assume so, but it's never been tried. Metroids work by absorbing the life-force of things, which is what prevents them from being infected, as their cells will absorb the parasite. Metroids and X are not really natural enemies, as the Metroids are bioweapons designed from the ground up specifically to kill X.
Bolwing up the planet they were on also worked.
This has basically all you need to know about the X.
0:27, Samus kills an X infected Hornoad, causing it to reform as an X.
0:32, The X outright ignores 2 missiles, then apparently bypasses Samus' armor without her realizing it.
2:02, Metroid's are revealed to be X' predators, and even their base cells can absorb energy. Something in this also turns Samus part Metroid somehow (presumably to prevent the Metroid cells from killing her too.)
On the space station, there were very few X infected specimens, but they took over the entire station extremely quickly. At one point they attempt to change the simulated environments to more suit their needs, implying they at least have some requirements. But otherwise, they just eat all the creatures on the station and proceed to violate most natural laws, conservation of mass among them. They seem to have no problems in magma, acid, or sub-zero environments. At one point an X-Parasite mimicking Samus and an Omega Metroid face off in a vacuum, so neither appears to need atmosphere, either.