So in my story the aliens break off from Earth a few forces to investigate a signal leading them to Equestria. To make things more interesting I'm planning to have them on the way add a new unit to their forces, a human 'alien' unit. Tell me what you think.

Description:
Size varies with natural human ranges for both male and female. Has cybernetic ruby red 'third eye' in forehead. A flat brestplate grafted to their chest, Left arm replaced with plasma cannon capable of long range fire, and legs replaced with quadruped cybernetic limbs. Due to human's potential for 'the gift' the elite versions are capable of spawning the 'Telekinetic field' ability.

id say take out the plasma cannon and legs, and perhaps make them psionic commandoes. A metallic outer body with only the essential organs inside, like a muton in a way but smaller, faster and with psionic abilities since that was one of the reasons the Ethereals wanted to enslave humanity.

So psychic slave warriors, then? Problem with psychic powers is that they normally go hand-in-hand with mental interference immunity. I'm not sure what to make of the concept.
As for massed hordes of mind-controlled human cannon fodder... I can see a specialised unit being bred for the purpose.

Or perhaps you could have Sectoid/human hybrids. In a lot of ufo stories people claim dna was taken form them and a few have said to see humans that have hairless heads, long bodies and black eyes like grays. So maybe its a human version of a sectoid, capable of the same abilities of the sectoids but a better fighter that would be about the level of an infantry soldier, not the deadliest thing out there but still a threat if not handled properly, (and better than a sectoid with a wimpy plasma pistol.)

Also this Is a pretty good idea to have human slaves with the aliens, since it presents several options such as, assaulting the slave ship(s) and liberating any living humans inside (some possibly being soldiers and replenishing the Xcom ranks)

Also a possible attempt by the aliens to use the human lookalikes to frame Xcom. Lots of possibilities.

1639201
I was thinking mechanically lobotomized psychic warriors, one of the things I noticed about the aliens is they don't seem to have a long range option like humans do. I was thinking of having them work in pairs, and possibly as sniper/assassin teams.

Though having them look more like a normal human for frame jobs...that has possibilitis too. Thanks Sneaky.

1639241
I think the "Aliens Harvesting Human DNA" would probably be involved in the creation of the Thin Men.