I don't really know anything about Mastiffs but, I have owned a Rotty and known a lot of them.
Mine was a "standard" and weighed about 85 lbs. Not one of the overgrown 150 pounders some are fond of.
She was even tempered, great with kids and friends, very gentle when rough housing (she applied the appropriate force for whoever she was playing with) and a big lap dog. Everyone who visited the house loved her.
We had a little mongrel (about 25 lbs) and a blond German Shepherd when we rescued her at 18 months old. Her and shepherd (male) got along fine from the start.
The mutt, who thought he was king of the roost, attacked her. We figured him for dead but, faster than we could blink, she flipped him over on his back, had her paws crossed over his chest and gave him a deep growl. Then let him up without a scratch.
I figured this was just a fluke, but then he attacked her again and she did the same thing. After that they got along great.
That said, she was very territorial. If you didn't belong there, you better leave, quick.
Once you were out of her territory, you were fine.
I have seen the same behavior in most of the Rotty's I've known.