Introducing a second dog in to my home...?

[Erica A]

Hello Dog Section.

I am considering adding a second dog in to my home for the last four months or so, i am going to be adopting from my local animal shelter if i do decide to add a second dog - I believe since they do a ten point behavioral evaluation on each dog that this would be my best option due to having another dog and i need to be sure that i will be getting a dog that hasn't shown any dog aggression tendencies.
I have a three year old female American Pit Bull Terrier (She isn't spayed due to the complications she has with anesthesia, she almost died twice when she went under anesthesia and they had to reverse the medication immediately.). My dog hasn't had any issues with other dogs in the past or present, she lived with two small dogs and a medium sized dog (All spayed females) for two and a half years and she meets dogs constantly when she is out in public with out any issues. She has been heavily socialized with other dogs since she was four months old (after she received her vaccinations.). My dog is some what submissive around dogs that are bigger than her.
I would like my second dog to be a large to extra large, adult (two to five years old), neutered male dog but would it be too risky to introduce another adult dog in to my home with my adult dog? Should i go with a puppy instead? My dog isn't very active, which is why i was considering going with an adult dog.
I have set my mind on getting a neutered male dog as i have heard that it is easier to introduce an adult male and female pair, rather than a same sex pair... Is this true?