getting an older, aggressive, protective dog to accept a yappy, friendly and playful one

This is a cross-post from askreddit.

After breaking up with my girlfriend, I may be forced to bring my 2.5 year old dog home. The only problem is that we have a dog - an 8-year-old Akita, who is very aggressive. He is protective and doesn't accept many people outside of those he grew up with. He listens to us fairly well, but tends to "get in the zone" and block everyone else out when he wants to bark or attack.

He has bitten us a few times out of reaction, and immediately expresses regret.

While walking him (he walks fairly well otherwise), we have to keep him away from other dogs and people. Once, he had a plastic "command collar" which pinched him as he pulled. He walked well enough, but a kid on a bike about 10 meters behind us must have scared him. He ran towards the kid and the collar broke like it was nothing, and our dog stood beside him and barked at him. He didn't once move to bite the kid.

He doesn't have a lot of exposure to other animals. There were a couple pet hamsters which he would sit and watch for hours on end - not sure if he wanted to eat them or not. There used to be two cats living here. This gives me hope - at first he hated them, but now he absolutely loves the surviving cat (no he didn't kill the other one). He knows of him as his friend and absolutely loves to see him and still looks for him in the house on occasion. There was also a pug, who is the dog of a family friend, that he liked. He used to follow her and sniff at her and generally annoy her. That is it for animals.

The other dog, for comparison, is a complete mutt - probably some of every type of dog. She is incredibly friendly and loves everyone. She barks at the other dog often (we keep them in separate rooms when she is over), but her tail is always going. She has had a few "dog friends" which she has played with. Besides that, I am not sure she could really do anything to hurt the big dog (male) anyway.

I tried talking him for a walk with her. My brother had him outside and I came shortly after. He saw me, and as I said hello to him he was wagging...and then we brought the dogs closer. They sniffed for a bit, and then the big one made a move to bite at the little one. I pulled back, and we did it again. This time, he ripped the leather leash like nothing and again went to bite her (and would have, if I hadn't pulled her back).

I get that he has confidence issues and frightens easily, but there has to be a way here. Can any dog expert Redditors help me out here?