Something very important to think about if you're going to do this- make sure the helmet fits snugly with the beanie on your head. make sure that the beanie doesn't allow the helmet to slide around on your head- do not buy a larger sized helmet just so you can fit a beanie underneath it- speaking from experience, spending months in the hospital and brain injury rehab because your helmet was improperly fitted or couldn't protect you adequately due to the way you were wearing it is not worth the 3 extra steeze points you get from rocking the helmet beanie combo all thuglike. get a ballerclava or something thin and wear it under a helmet that fits
also, sidenote, get a helmet that meets industry standards, not a hardhat. the hardhats look sick, and they're better than nothing, but the amount of money you save on the helmet is nowhere near the amount of money you'll pay for say... an airlift.