People do it all the time without problems, but any time you have a small breed living with a large breed you have a chance of the small dog getting hurt. Sometimes this can be intentional and sometimes accidental, but with one dog large the small dog loses.
It would be important if you decide to go forward with this combination that you watch them very closely and monitor their play etc. I would also recommend the Rottie be of the opposite sex from your current dog. Opposite sexes have a tendency to get along better together.
I had a great dane with my pugs and can honestly say I will never mix large with small again. My experience was not good, but all dogs are different and you may have no issue. I know many on this formum mix large with small with no issues.
And lastly, MAKE SURE your rottie comes from a reputable breeder that produces dogs with stable temperaments which will help your situation greatly.