"Are you Klingons?"
With a couple of rare exceptions, the plural is not required, even when plural, and it is also not wrong, even when unnecessary. It is not grammatically incorrect to say tlhInganpu' tlhIH, it is just overly clear. Mostly, in this course, we leave the plural off when other parts of grammar make the plural clear. But that is not because it's incorrect, rather just to help the students of this course get used to that usage. Similarly, we've allowed a couple that don't need the plural suffixes to use them so that students get used to both ways.