Could I sub rice for the couscous ?

Sure...but keep in mind that certain grains take longer than others to cook. Rice will take longer than couscous, so you just have to make sure your chicken doesn't overcook.

Of course. You can sub any grain you want. Both ideas sound good to me.
Cheers!
i

This is not an answer, but I was similarly wondering about subbing in whole wheat couscous or quinoa