My 4 kids range in age from 22 to 10. The older ones frequently tell me that I was harder on them. And, quite often, I was - I usually had to tell the older kids something over and over and over. That means that by the time they listened, I was really angry, and an afternoon without TV had become a week without TV. The younger ones grew up watching the older kids, and they push the boundaries less because they know that when I say something, I mean it, and complaining or ignoring me will make the punishment worse, not make it go away. (The oldest kids, only a year apart, always hoped that if they complained enough I'd change my mind. Never happened. ;P) I usually only have to tell the younger kids something once.
Just repeat what you've already told your oldest daughter: you are the decision maker, period. Remind her that some day soon, she'll be the mom, and she won't want you meddling with the grandkids. Also point out that every person is different, so you'll often treat them differently. That's not a bad thing, because people are individuals.
As for your youngest, stand firm on your rules. Lots of people will tell you it's no big deal, everybody experiments with pot, it might even be a good thing, but DON'T believe them!!!!! My best friend is happily 3 years sober now, but lost everything - job, home, family - to drugs. He now tells everyone, "There is no such thing as 'experimenting.' You use or you don't. Make sure you don't!"