If mogwais only become docile after living for 100 years, how did Billy's daughter have one that was (presumably) calm?
I mean, I guess this is assuming that the Mountain Dew commercial follows any kind of continuity, or that the show and movies share the same continuity. But for all we know, they just might.
But that also gets me to thinking, Lenny wasn't particularly evil, he was barely mischievous, and he was a newborn.
The novelization explanation would make more sense, that it's just really really rare that they're docile, and since Gizmo's multiplied so many times it was bound to happen eventually.