My sister just purchased her 3rd Doberman puppy about 3 weeks ago. Bella is from the same breeder that they purchased Star from 8 yrs ago. Both Bella and Star get along with all of mine just great. Prior to them, there was Jordan also a Doberman and she was probably the best dog EVER! I have to agree they do need a lot of exercise so be prepared to provided it.
I did not have my pack when they had Jordan but my boys were small and she was always very good around them. Basically, they all grew up together.
They purchased Star from a breeder is Wisconsin and she is even more laid back than Jordan was. My BIL started to look for a different breeder when they decided to add a new puppy and I told him that he should go right back to where they found Star. He tried to argue that it was a long drive
I told him that to have a dog like Star and know the temperament and breeder why would you want to take a chance by changing to someone you knew nothing about.
As for having a small and large dog in the same house, they also owned a Dachshund and they lived in peace together for many years. When they owned Jordan they also owned a Westie and there was never a problem between them either. Now that Westie was MEAN and was aggressive to everyone that walked into our house when I was a kid.