How do I introduce a puppy into a pitbull mix home?

Question
We recently came to the conclusion that our dog Lucky, pit mix, needed a playmate.  We discussed it and Lucky never showed aggression to any dogs except for the occasional playing.  We thought it was time and ok to introduce a three month old shepherd/sheltie mix.  We let the puppy roam the house and get comfortable with her surroundings.  Then we brought lucky inside and calmly introduced them.  At first we held the puppy and did it that way.  then slowly we put the puppy on the floor and Lucky got excited at first but then would not let up on pulling away.  We put him back in the garage and an hour later tried again.  This time when we put the puppy on the ground he growled.  I don't know if it was a play growl or aggressive growl.  We didn't want to risk it.  He is a pitbull mix.  We adopted him from the pound in October.  So he was here first.  We however were not told his background now that he was a pit mix. His papers said boxer mix.  I do NOT want to get rid of Lucky because this issue.  I really want this situation to work.  I have grown attached to Lucky and want to work on this.  My Fiance had promised that Lucky would not be an issue when then brought this puppy home.  I was skeptical about the puppy.  Also when we first got Lucky skeptical about him being around other dogs.

Do you have any type of suggestion or advice?  Anything that will help this situation and make this a loving home for these to dogs.  They are both on their second chance homes.

Thank you for your help.

Answer
Here are some articles that should give you some good suggestions:


http://www.marinhumanesociety.org/Behavior/Handouts/MultipleDogs.pdf
http://www.ddfl.org/behavior/dog-intros.pdf
http://www.ddfl.org/behavior/canine-rivalry.pdf

http://www.marinhumanesociety.org/Behavior/Handouts/PreventingAggression.pdf
http://www.marinhumanesociety.org/Behavior/Handouts/BreakingupDogFights.pdf
http://www.marinhumanesociety.org/Behavior/Handouts/CanineConfrontations.pdf

http://www.marinhumanesociety.org/Behavior/Handouts/DifficultDog.pdf