Headspace

I just racked a 5 gallon batch of mead from my BMB to a glass carboy. It left a bit of headspace in the carboy. I remember reading somewhere that I need no headspace in my carboy, and that I could add a similar mead to make up the difference. I have a gallon of mead used a different yeast and honey that is about 5 months old. Should I add it on top?