How to introduce my dog to a new dog I am getting?

[Jas]

I have a 4 year old Pitbull/Pointer mix and I am going to be buying a 3.5 month Rottweiler Puppy. The Pitty/Pointer mix is very very aggressive with other dogs and is very territorial and he is not neutered but I am going to get him neutered hopefully in the days to come, will that help? How can I properly introduce them together? Last time I introduced a LAB to my dog, they were both on leashes but after they sniffed each other it turned into a big fight, which then thank dog to the leashes was broken up. So how can I introduce my dog to the 3.5 Month Rottweiler?
But the dog is only aggressive to other dogs and very very nice to its owner