Thinking of a second dog

Hi , sorry for long post but ill try get as much info here as possible .
we are looking to get another puppy , the puppies are 9 weeks old now and are in a rescue , both parents are know , mother is an American Bulldog the dad is an American Pitt Bull Terrier . As you might remember our own dog Missy is just over two years old and is a Staffy Pittbull cross , she is a very sweet dog and is kept indoors as purely a family pet  as will the new puppy be , Missy can be excitable with guests and when playing on walks in the park , she can be a little  ball protective but other than that she plays well with other dogs , in fact she mostly ignores them .
So ... what steps can i start taking for as smooth an introduction as possible , are there things i can do now with Missy to help when the new pup arrives , i Know im going to have to up my game with training as two is double potential trouble and i need to protect them .
Lastly im getting mixed opinions on weather another female or a male would be better , they will be left alone for up to 5 hours on rare occasions , but will be well exercised etc .Any and all advice is greatly appreciated .