Welcome Welcome to R2R!
I do not know what cuckoo fish are exactly.
Everything seems to be getting renamed lately but sounds like a cichlid, maybe?
Pictures would help identify.
Moonlight gourami, another misnomer, maybe pearl gouramis usually can take care of themselves, especially if in a pair.
Not sure if you are asking if the gouramis will eat the cuckoo fish or the other way around.
Gouramis will nip at times but if you keep 2 together they'll usually concern themselves with each other.
Anyway, love to help, pictures are great and welcome welcome to R2R!