[ColleenSwerb]

The dogs are fighting for dominance in the pack. They both want to be "top dog". While part of it is just living with it and letting them work it out on their own, there are some things you can do yourself.
1 - Decide who is going to be the top dog. That doesn't necessarily mean that your Scottie automatically gets the spot though. If he's normally a submissive dog, and the other dog is more dominant, it would be fine to set the foster's dog above the Scottie.
2 - Top dog does EVERYTHING first. Eats first, gets pet first, gets to go outside first, etc etc etc. Make them both sit down before their meals (I'm a firm believer that a dog should always have to "work" for their food, even if that is only sitting until I say they are allowed to eat, or shaking my hand. It's still work to them.) and then only put the dominant dogs bowl down first. The other dog must sit patiently and wait until the other dog is done eating before they are allowed to eat (or, once they get better, you can put their bowl done while the other is still eating). Make sure you have control of the other dog so the first dog can eat in peace.
3 - If they fight over toys, no more toys. If they fight over table scraps, no more table scraps. Eliminate as much stuff that they fight over as possible, and it will help them to settle it on their own.

I'll add more things as I think of them. You'd have to enroll both dogs in the training class and have someone else with you to help do it. Personally I think you can do it on your own. Go to the library and check out any of Ceaser Milan's books (The Dog Whisperer), they're really wonderful.