Heat olive oil in a large skillet over medium-high heat. Add sweet potato and onions. Cook, stirring often, until potatoes and onions start to brown, about 5 minutes. Reduce heat to low and cook, stirring often, until sweet potato is tender, about 15 minutes. Add chili powder, oregano, garlic salt and pepper. Toss to coat and cook 1 minute more. Stir in chicken and 1/4 cup water. Let cook about 5 minutes.
Toss cabbage with lime juice in a large bowl and season with salt.
Place tostadas on a sheet pan and warm in the oven for 2 to 3 minutes. Spread a couple tablespoons of refried beans over each tostada and top with chicken mixture and cabbage. Serve with hot sauce, if using.