Idea about the "splitting alien":
Biomechanical (a thing that has both biological and mechanical parts, correct?) alien that has four or so smaller drones that it can "lay"/deploy and that will automatically deploy on death. I'm not terribly sure of what the main alien would do but I think it would end up as being the "tank" of a team, as well a having a (somewhat weak) medium-to-long range attack. Something you wouldn't be terribly afraid to go against but the damage would add up quickly enough that seeking cover would be preferable.
The main feature of the thing would be the four (possibly replaceable, more on that further) drones, something akin to somewhat smaller bloodspiders. These guys would instead be glass cannons: weakly armored, fragile but hellish things if they get close to an agent. They could also act as spotters for snipers when the AI permits and would be hard to see once/if visibility gets added. In case the host alien ends up 2x2, they would also be suitable for house cleaning. Their main attack would be fast melee attacks that would add up quickly if TUs permit.
They could also be replaceable whenever the host has less than four under it's control, somehow. My current ideas would be to either spend a full turn or two to refill or eat a corpse and then spend a turn.
When the alien dies, the four drones would be released, somewhat damaged and berserked. As such, they would simply rush straight at the PHALANX while ignoring cover. One twist that could be added is that since the host would be their link to the XVI, when they get released on death, they berserk against anything in short range, whether PHALANX, civ or fellow alien. Thus killing the host would be risky as it can take a lot of punishment but managing to do it in the middle of the enemy would leave them badly damaged, giving you a nice advantage. It's risky, because the enemy could just flank you or simply rush you while you try to shoot the damn thing.
The background/lore could be something about an alien race the XVI modified on a grand scale to fit a better job inside the hivemind or that they modified genetically one of their existing aliens races into another breed, this guy.
The mechanical side is there to cover the possible illogical side of keeping seperate aliens inside of the host.
If you want, you can think of this guy as somewhat like the Half-Life series' poison headcrab zombies. Not much, but the same "tank host that has smaller glass cannon/fragile speedster units do most of the dirty work" combo would be in effect.