Canon that the Xenomorphs were created for some reason, so it stands to reason that eventually the Powers that Be (like the Space Jockeys) (or one of their researchers) would try to go in this direction. However, a few problems cropped up:
Sil and Patrick are not the same type of creature.It would explain certain inconsistencies. Like why Patrick's true form doesn't look anything like that of Sil's son. And why Sil took much longer to develop from fertilized egg to baby than seems normal for her species (she was presumably implanted in a human surrogate mother, and her species doesn't have whatever mechanisms allowed Patrick's offspring to just suck 7-9 pounds of material out of their hosts in minutes, so her early growth was dramatically slowed down). They are related but distinct species (or subspecies - they're obviously interfertile). This theory is also appealing because it removes the third movie's implication that any offspring Sil had would probably have been sickly and sterile so she could never have been a serious threat to humanity. And it would provide a handy excuse for any fanfic writers who might want to throw out some of the things we learned about the creatures in the second and third movie.
The aliens are a seed/specialized unit of a superorganism.The aliens are fast-maturing, fast-breeding creatures which seem to operate largely on hard-wired instinct, which isn't a life strategy that one would typically associate with selection for high intelligence. It would make sense, however, if they're specialized autonomous units designed to serve the purposes some larger superorganism, something like the Tyrannids or Zerg.
The aliens were just misunderstood.They didn't plan Genocide by Supermodel at all. Perhaps they're very keen on genetic modification and given how resilient alien Sil is, they thought they are giving us a gift. For them it was like sending a machete and a hockey mask, because, hey, a machete is a useful tool, and everybody likes hockey, right? Right?
The aliens were curious as to what we would do, to see if we really were a threat.The aliens wanted to see how we'd react to the organism they sent along with the methane formula. They may have reasoned that their first gift would make us curious to try out the second, so we would naturally try to create and raise Sil and her kind. What would happen after that was the critical thing. Remember, one of the scientists speculates if the aliens consider humanity an "intergalactic weed" to be dealt with, and if Sil is meant to confront us. How we deal with her, whether we fight her, tame her, or fall to her, will reveal to them what we're capable of as a sentient species (and possibly how to be further considered.)
The aliens grow using the same method as plants.As You Know plants absorb CO2 from the air and release oxygen. They use the retained carbon to grow and thus they acquire most of their mass from the air itself rather than absorbing it through the roots, as one might expect. The aliens may gain mass and regenerate with a similar process, just sucking carbon right out of the air as they grow. This also explains any other conservation of mass problem with aliens in sci-fi.
The hot chick who boinked the guy to death in the men's room in Species 3 was the daughter of the 2nd woman in Patrick's threesome in Species 2Look at them and tell me they aren't related.