ماذا عن "we are going to assume that they are couple" ؟!
It is correct, but you have to add an "a" before couple.
couple means two pairs, so its plural.
why to add an "a" ?
couple is "a pair", which is an example itself! "a pair"
i mean that couple is plural word in meaning, why do we use "a" with it ?
Couple is the same as pair/group (called collective nouns). You can say "We are going to assume they are a group/pair."
we are going to suppose that they are a couple