You can use whatever leftover cooked meat you have. Steak, roast, anything you have. Whatever you use, I recommend keeping the filling pieces chopped pretty small so they fit well in the tortilla shells and everything in the filling is a uniform size.I always try to include a veggie in the beef mix, and this time of year it's zucchini. Any other mild-flavored vegetable will go great in beef tacos, the beef and the seasonings have bold flavors so slipping some mild-flavored veggies in there shouldn't bother anyone.I actually make a large batch of the spices in this recipe so I can have homemade taco spice mix on hand. I add about 3 tablespoons of the taco spice mix per pound of meat, but taste test to your preference. Here is the spice mix recipe for a larger batch: Homemade Taco Seasoning