Celtic Frost while mainly an early black metalthrash act they did do one whole hair metal album Cold Lake is often cited as their stab at it by multiple reliable sources examples Sound of the Beast Spin etc I personally believe that one out of six albums is enough to qualify them for the list though we may need more editorial opinions on issues like them As for Helloween I agree that there is no way they are a hair band but Chuck Klostermans book Fargo rock city a heavy metal odyssey in rural Nrth Dakta specifically includes them on his run through of hair metal groups on page  I believe he always mentions other controversial examples like Accept but I cant be sure of this because my time on that page expired on Google books  As of now you and me are the only people really getting involved with this project Hopefully other users will get involved in this debate