In a traditional couscous, meat is only optional. But when you use one, you have to use only one sort and not mix them (sheep, goat, chicken or even camel guts) so the broth can perfume the couscous grains while you cook them with water vapor. You would never use fish, or only, in some cases, very firm-fleshed white ones like grouper or sea basses (in southern Tunisia or in Algeria).
What really matters are the vegetables you use. Traditionally, it's zucchinis, onions, carrots, turnips, chards, and of course chick peas (chick peas are cooked separately with couscous). But then, Tunisians also add peas, coriander and red, hot peppers (to make the masfouf), while Moroccans prefer a sweeter (less hot) variant, sometimes with dates, grapes, almonds and cinnamon. And in Algeria it's something between those two extremes (hot vs sweet).
Tunisians also prefer to serve the couscous with marka, on a separate small plate. Marka is a kind of puree made with tomatoes, some of the broth and (again) red hot peppers.