Welcome to the board! I'm Kristina and have 2 Pyr pups. I'm sure you'll get some great advice/tips to stop the biting or mouthing. Different methods seem to work differently depending on the individual dog from my experience. My last dog before these pups was an American Eskimo and she started nipping/biting as soon as got her home at 8 wks and it got increasingly worse, quickly. I scoured the internet for advice on how to help it and nothing worked. You name it, I tried it. The yiping didn't even stop her for a second. Because of her breed, she was an excellent jumper and got to where she could jump up to my stomach and would bite as she jumped, it was a pretty bad situation. If I had had the time then to get her into some obedience or something, I would have and regret not doing it but her breed are kind of spazzy so that didn't help matters.
With these pups, I started with telling them 'no biting' from the beginning but it didn't seem to be working. I then discovered bite inhibition and it made a lot of sense to me so I started that. I then read conflicting views on it and decided I wanted to hault the mouthing altogether. So when they mouth or nip me at all, I put my hand around the top of their muzzle and curl their lips over their top teeth and apply pressure. As I do that, I also say NO or NO BITING very firmly and that's generally all it takes for them to stop, like a light goes off and says oh, guess mom doesn't play that way. In the past they would lunge at my foot or something while I was sitting and try to engage me in play and it would HURT..so I'd yell in pain and that would get them to stop dead and move far away from me...just like the pause that comes after one pup yelps when another bites too hard. If none of that works, I walk away from them. It may work better because they have eachother and go get distracted easily, but this breed also doesn't seem to stay interested long in any one thing, thankfully haha. If your pup is really overly excited and nippy, you may give her a time out in her crate (if you are crate training) to settle herself, and signal to her that if she plays that way, it's the end of play time.
The things that are working for them though, did NOT work on my last dog..she would just chase after me, there was no controlling her- at least nothing I found to be successful at. I don't know anything about Rotties but I *think* most mastiffs are pretty laid back so hopefully you won't have to deal with this long and your pup will catch on and start responding to something.
Mine are doing excellent fortunately and when they do end up mouthing, they at least have a soft mouth but I still don't want them doing it. When they start to be in the throws of teething is when the real fun is going to begin.
GOOD LUCK!! Another joy of puppyhood.