There are actually a few forum members that I can think of that have Rottweilers and Pitt Bulls & they are completely fine together. Maybe they'll chime in here.
If it was me getting the dog, I'd wait. We're interested in possibly adding a female to our family once Julius is three. I would just concentrate on making the current dog as well trained as possible before adding another dog into the mix. I know that at 10 months old Julius was still kind of in his "angel" phase.
I thought he would stay that way & although he is the sweetest dog he has done some things to test us (not wanting to heel after doing so perfectly for awhile, etc.) & we had to kick his butt back into daily training to deal with that. I don't know what I'd do with another puppy.
He doesn't want another dog to play with although that would be fun. He wants your attention.
:)
Brooke