Yes, seitan would be classified as a grain.
And the "protein complementarity" theory, which was espoused by Frances Moore Lappé in her "Diet for a Small Planet," and stated that you must eat complementary proteins together to get 'complete proteins," was later proved to be erroneous, and Lappé corrected herself in later writings. As long as you are eating a reasonably varied diet, as Pete stated, your body will store extra amino acids (the body breaks down proteins into their individual amino acids, then rebuilds them into other protein structures according to need) and mix them with incoming amino acids as needed.
So, instead of having to eat beans and rice at the same time (though they do make a nice couple!) you can eat rice for breakfast, and beans at lunch or dinner (within about 24-36 hours of each other) and the body will have the amino acids it needs.
If you are still concerned about complete proteins, foods like soy, hemp (one of the best sources of vegan protein, IMO) and quinoa contain by themselves amino acids in the balance needed by the human body ("complete proteins").