To Add a New Pup...?

I've been considering adding a new puppy to our home but I have concerns on how this will effect the "pack" I already have. We only own one dog and I was wondering-- is it very difficult to jump from one dog to two dogs??
I've heard that some dogs will learn bad habits from the other dog-- any truth to this?
There are plenty of people with multi dog homes and I was just wondering what the pros vs cons?
BTW-- after a lot of research on a reasonable family dog for young kids we decided on the Shih Tzu or the Cocker Spaniel to add to our home IF we decide to make that leap!