Old dog - new puppy - how to avoid problems

Question
We have a 4 year old Basset- Bella- and she is the sweetest, most spoiled dog in the universe. We decided to get another one and the new Basset puppy is going to arrive in a couple of days ( another female). The closer the time comes the more we are worried that the arrival of the new puppy will change the personality and the behavior of our Bella. What can we do to assure that the integration of new puppy will be as smooth as possible. We could not bear the thought that Bella might loose her sweetness and her quirkiness and become a different dog altogether.

Answer
It would have been MUCH BETTER to temperament test your new puppy.  I'm seriously HOPING you did NOT buy from a pet store or some puppy mill breeder.  Your new pup's breeder really should have asked you to bring Bella around and you really should have had the opportunity to meet the pup's dam (mother), see a five generation pedigree and determine whether or not the pup you liked the most was bid-able, not too dominant, etc.  Buying a puppy unseen is A HUGE mistake.  It also would have been a much better choice to get a male.  

However, that being said, you must treat the new pup as you would any single dog by way of carefully, and fully, socializing her to everything outside (children, people, dogs, moving cars, whatever is in your normal environment) by herself (without Bella.)  Meanwhile, Bella will most likely ignore this new pup (perhaps for days) and this is normal.  Respect her need to give the new pup a strong signal of rank and dominance, that's pretty much what's meant when one dog actively ignores another.  Don't worry about the pup's "feelings"; she is NOT Bella's companion, but yours.  Show Bella every courtesy as the top ranking dog; she should be shown attention first, fed first, played with as usual, taken out with you for car rides or walks WITHOUT the puppy, etc.  If Bella sleeps with you or in your bedroom, continue that but don't bring the puppy into your room (until rank has been firmly established, several months down the road.)  Put the pup on a house tab (long lightweight training leash) to keep her out of trouble, establish YOUR rank and assist in house training.  This will also give Bella  a very strong signal that her rank is higher than the pup's.  When the pup first arrives, fight the urge to fall all over her for three reasons.  First, the pup will be frightened (if she's coming by air and she's 8 to 12 weeks old, this is not good, she will be in the middle of a strong fear phase and giving her comfort but not fawning all over her is the way to go.)  Second, while you won't be actually ignoring her you won't be overly attentive, which elevates your rank in the pup's eyes and which will make a statement to Bella.  Third, Bella will have a day or two to adjust to the pup's presence, observe your attention is still on her, and relax a bit.

Be sure you take this puppy to the veterinarian within the time alloted by your contract with the breeder.  Maintain Bella's lifestyle as a pampered pooch, build your trust and affection bonds with the pup by treating her with affection and consistent patience (as well as positive reinforcement training), and it will all probably work out.  However, never purchase ANY puppy in future without seeing the entire litter, meeting the dam and extensively interviewing the breeder in his or her home.  Good luck and if any further questions, repost.