There are a few really easy ways to add protein to potato tacos.  Beans are one of my first suggestions.  You can heat up some vegetarian refried beans and spread them on the tortilla before adding the potatoes.  You can also add cooked pinto or black beans to the potato mixture while it cooks in the pan. Quinoa also works great when paired with potatoes.  You can mix in some cooked white or tri-colored quinoa with the potatoes while they are cooking. Tofu or tempeh can also be mixed with the potatoes.  You can keep it simple and simply chop it up and add it to the pan with the potatoes or you can also make a tofu scramble to serve up as a base for the potato tacos.