Combining polyiso and mineral wool for class A fire rating
It’s complicated. Actually, the Code says “foam plastic insulation” must meet a flame spread index of 75 and a smoke development index of 450. (ICC R316.3) However, there’s an exception for loose fill insulation and insulation more than 4” thick. 

So, if the product you are using exceeds these values, you can cover it with various materials, including 1 1/2” min. thickness of mineral fiber insulation. (See ICC R316.4.3)