RE-training a 4-1/2 year old Toy Fox Terrier?

[donnav]

Is it possible to train a dog at this age to get him to change his behavior problems. (Jumping, scratching, barking, low level signs of aggression). My sisters dog is very much an Alpha & rules the house due to my sister raising him like an infant. Unfortunately, now there IS a newborn infant in the house & "Zane" is making it difficult to enjoy this time without his disturbing annoyances of showing his dominant place. It is not constant, but happens often enough that it is of a great concern to me that his aggression may get worse. I bought Sit, Stay, Fetch for help because we love "Zane" & want him to be as happy as we are with the new baby, but I am concerned that trying to get a 4-1/2 year old to change his patterns at this age may make him more of an aggressor due to the confusion he may encounter. Should I try to get him RE-trained with the help of this book, or is it a lost cause at his age?

[prespalm]

We have 2 dogs (cocker (20lbs) & beagle/mix (20lbs). both are about 3 yrs old. Recently they are exhibiting extremely aggresive behavior toward cats. To the point of attacking them as a "team". We wonder if getting a kitten will help their attitude toward cats in general?