Pollo Ranchero is made with poblano and jalapeno peppers. Poblano peppers have mild heat and jalapenos are a bit hotter. You can easily reduce the heat level of the dish by swapping out the jalapeno peppers for bell peppers or just using poblano peppers. Alternatively, you could increase the heat by leaving the seeds in the jalapeno and/or adding an additional pepper like a serrano pepper.