Can I wear a beanie under my freestyle helmet for extra warmth?

You can, but it requires careful attention to fit. A thin beanie (2-3mm thickness) can work if you size the helmet to accommodate it, but thick beanies will compromise the helmet's ability to protect your head by creating an unstable, shifting layer between the helmet and your skull. If you need extra warmth, consider a helmet-specific thin liner or balaclava instead of a bulky beanie. Always verify the helmet fits securely with the beanie — it should not shift when you shake your head vigorously.