Just a word of warning. Rottweilers can be same sex aggressive. Two females that have done well together, can one day decide they hate each other. Watch when they are starting to get too rough, or serious playing. Things could turn ugly very quickly!!
I'm curious about your 2 youngest dogs pedigrees. You mention that they are German. What breeder in Germany are they from?? I love looking up pedigrees, and bloodlines. If the sire and dam are from Germany then your pups come from German lines.