Yes, you can bake the focaccia in an 8" X 8" metal pan or even a 9-inch round cake pan.  Metal pans work best because they get hotter helping to cook the focaccia evenly. I typically use a cookie sheet because it provides a bigger canvas for making bread art on top of it and because we like the crispy bottom and edges of the focaccia.  Baking it on a cookie sheet gives you more of those delicious edges.