Help with Rescue

Hey guys!

My fiancee and I are currently working with our local Humane Society in their Foster-to-Adopt program. Our new dog, Regi, wasn't taken very good care of by his past owners and sadly contracted heart worms. However, his last appointment is Monday, the 18th, when we will officially adopt him as our own. Because we can't get him too excited until the last appointment, I haven't been able to take care of certain issues that he has. However, once he goes in for his last appointment, I'm going to start working with him to hopefully train out his problems.

Regi is a Chinese Crested/Jack Russell mix and is 3 years old. He is fairly house-trained, but has had a few accidents recently.
His big issues, though, are the following:
Some separation anxiety(his past owners never left him alone).
Territorial dog aggression(he loves people, but when another dog walks by, he will freak out and not stop barking)
Trying to bolt out the door for my fiancee(Regi never runs from me, but has bolted out the apartment door for my fiancee leaving him chasing the dog around the complex)
These are his three big issues. I REALLY want to try and train him out of the territorial dog aggression since my fiancee and I would like to adopt another dog in the future when we have a house and a big yard.

I was curious if any other members may have some advice in how to train these problems out of Regi. I would like to use positive reinforcement techniques.

Thank you guys!