Since you have a Newfie, I'm just going to GUESS that you are probably well aware of the needs of large breed dogs and already know that this cross is gonna be a big one!

My vet has this cross. It's a wonderful dog. It's also very beautiful and well proportioned. His head is huge! It's not as large as you might expect, though I'm sure some other crosses of Rotts/English Mastiffs are much larger. His dog is a 140 lbs male--about as tall as a large Rottie, and very compact. His dog is very nicely tempered, having inherited both breed's better angels. Obviously you can't necessarily assume that all crosses of these breeds will be as docile, since both Mastiffs and Rotts were bred for protection. You probably know that!

He frequently brings his dog to the clinic, where the dog likes to lounge and seek affection from clients. I must say, I'm crazy about this dog! I just marvel at what a beautiful, gentle monster he is.

My vet also has a female Bullmastiff. He says both dogs get along well. He's a vet though, and very anal about training--but he's an example I know of where two powerful dogs live in peace with each other. Then again, your dog might turn out to be completely nuts. LOL!

I hope this helps you a little.