Yeah, this is why I intitially made the challenge as hive vs hive or representative breed vs representative breed. The Zerg exist on a much larger scope than the Aliens, so as far as overall prosperity of the species goes, it's not much of a contest.
Although, it's pretty much impossible to exterminate the Aliens entirely, because they're sprinkled across vast regions of space where nobody's ever discovered them. Their origin is STILL a mystery, they just pop up more often now because we're starting to spread into their area of influence. They mostly move and spread by stowing away with other, more mobile species...much like lice and other bugs.
And just as the Zerg could infest Aliens, imagine what would come of a facehugger nailing a Zergling or a Hydralisk. Hence what I said about potential hybrids scaring the **** out of me.
Oh, and the Overmind wasn't directly controlling each and every Zergling across light-years. That's where the hierarchy comes in. Overmind > Cerebrates > Overlords > Minions. In spite of appearing in-game as buildings, the Overmind and Cerebrates are mobile organisms, just massive and generally staying in place unless forced to move. Cerebrates go where their main hive cluster is, and Overlords accompany the various underling strains. Kill off the Overlords and the Cerebrate will have a harder time controlling the minions. Kill off the Cerebrate and the Overmind has no command at all unless possibly they're in close proximity (relatively). Hence the Zasz fiasco.
So here's how I see it in terms of matchups:
Hive vs Hive, both fully developed. Slugfest, I'm curious to see it play out. My heart goes to the Aliens, but the Zerg would be a hard enemy.
Zergling vs any Alien breed: dead Zergling. A larger strain of Alien could mow down dozens.
Hydralisk vs agile Alien breed: fun fight, but dead Hydra most likely. Aliens just have superior speed and intelligence.
Ultralisks would probably require a few Aliens to take them down, but the minimum would be maybe 3. Once they got ON the brute, it'd be game over. A Queen would demolish an Ultralisk due to being so much faster and more agile. I mean, as big as they are, Queens are mostly spidery and quick as opposed to brutish and powerful. It'd run circles around an Ultralisk, in spite of how they lumber around in Extinction.
Airborn species of Zerg would fare better against Aliens, but the Xenomorphs are crafty. Eventually they'd probably figure out a way to get at them.
Probably the most effective ground opposition to a swarm of Xenomorphs would be a line of Hydralisks, but only in environment favoring the Hydras...probably an open, flat prairie. Any walls or trees or buildings and it starts favoring Aliens. Hydras would win British Militia-style, in lines mowing down waves of Aliens. All the Hydras in the Swarm couldn't beat encountering an equal number of Aliens in a system of caves or tunnels.
Unit to unit, Aliens are probably superior. I have to say though, an encounter between Zerg and Xenomorphs would be an amazing watch.