There is nothing wrong with topping up that would make someone not a purist, that is kind of a silly idea on how to make mead, but now you have some mead bottled up and you can use that as a topper and remain pure

You can also make 6 gallon batches to start out with and rack to smaller carboys as you go, but really, topping off is a valid winemaking/mead making step. Congrats on your mead! Also, you might want to fill the bottles up just a tiny bit more next time.
WVMJ