I think some races should be exempt from wearing certain helmets, lets face it, if you feel inadequate about the size of your horns, wearing falsies ain't gonna cut it. Likewise fins and other abnormal head protusions.
It would be as if those "Cat" people were to wear a pair of paints with out that whole cut out for their tails, how painfull would that be with their tail all crunched up under a par of tight fitting leathers. OUCH!. Well that's is just like that head ridge that those toons with wings have. And the horns, being hard nonbendable apendages they would not allow another nonbendable element to fit over and in place of them, like trying wearing two bicycle hemlets at the same time, no matter how much you pull the top one down onto your head, it will never conform to the the bottom ones state, as it is preconformed to your head size.