Ridley Scott's newest movies in that universe have muddied the waters quite a bit. I am pretty sure much of the newly established lore overrides anything from previous expanded universe stuff such as the graphic novels and video games.
Prior to Prometheus, my head canon explanation was that the space jockeys (engineers) were not related to the xenos, but had raised them and genetically modified them from an existing species into a bioweapon. An outbreak on that ship led to it crashing on LV-426 (whether deliberately or by accident is unknown but I lean toward the former, based on the warning beacon discovered by the Nostromo crew in Alien). Honestly, the original Alien tells us everything we need to know, albeit visually, and I find the "explanations" for the space jockey attempted in later media to ruin the mystery and aura of that original scene. The more aggressive nature might be from David having incorporated more aspects of the "raw" neomorph physiology and behaviors into his designs. David would've had no knowledge or material from LV-426 to work with (since the events of Alien haven't taken place yet at this point in the timeline)
When you speak of Grey Aliens, are you referring to the Neomorphs seen in Alien: Covenant? I assumed that they were either a different variant of xeno created by the engineers at one point, or that they represent the "rough" or more natural form of the xenomorph species prior to their modifications at the hands of the engineers (and later David). I also believe the xeno seen in Covenant was distinct from the Xenos in the other Alien films, as it was engineered by David based on his research on the engineers/spacejockeys' experiments rather than spawned directly from the original variant line seen in Alien, Aliens and Alien 3. Hence, it had a more sinewy, organic appaearance vs the biomechanical look of the original aliens. It also seemed a fair bit more aggressive and animalistic than the drone in Alien, which was cautious and opportunistic.
