Rules, no not really and the BATF won't come knocking if you mix your brews. But like beer or other wines there are style guidlines like those from the BJCP for meads
. The Mazer Cup
follows the BJCP guidlines for submissions. If you are just brewing for yourself and your friends do you really care? And why do all the mixing? Wouldn't it be more prudent to just brew a pyment or cyser and not risk the aeration or off flavors?