I really like the idea of adding Cleaners to the game.
I have some ideas:
Collectors reinforcements in UFO crash sites - In some UFO crashes, a cleaer unit (3-10 cleaners) would arrive at the scene after some time,which would come to rescue the surviving aliens. (Similar to what is shown here https://www.goldhawkinteractive.com/forums/index.php?/topic/35769-ufo-crash-sites/)
The presence of Collectors in alen base - supported units at entry.
Adding another type of high-ranking cleaner that would have an elongated skull (https://en.wikipedia.org/wiki/Artificial_cranial_deformation#Americas) - it could be a contact dignitary, or a cleaner with psionic abilities.