[Help] Advice on adding a second dog to the house!

Hi all - first time poster here.My wife and I are a young couple (late 20s). We have a 3 yr old cat who has the run of the house, and a 7 yr old Shepard/cattle dog mix who is only allowed downstairs. The dog (Sammy) has some behavioral issues, such as dog aggression, unpredictable territorial aggression to humans (when they try to leave she nips people very rarely and only if the guests are doing something I told them not to). We are very careful with Sammy, and I have hired 3 different Behaviorialist with no luck. She is also going blind from glaucoma, totally blind in left eye and the right side vision is being preserved through diligent medication.The dog and cat get along fantastic, they are infrequent buddies, but they will hang out in the same room no problem at all. My wife and I are considering getting a new puppy (she works from home 5 days a week) but she is very worried about the new pup picking up bad habits from Sammy. Specifically the dog aggression and Sammy's whining (she is very vocal and my wife has a low tolerance for high-pitched whines... it "scrambles her brains.")If we are diligent in training the new pup (who would be a dachshund mixed with either a fox terrier or a poodle), is it reasonable that we could prevent it from picking up those behavioral issues or bad habits? Or am I being silly in thinking we could stop Sammy from rubbing off on a new puppy?Thank you in advance! via /r/dogs http://ift.tt/2rHodR8