This is exactly what is going on with Mickey and Molly right now.

Let them fight it out. As long as there is no blood, sit back in your front row seat and watch.

As for stealing toys, do NOT allow the new dog to play with your old dog's toys. We give Molly a tap on the nose and say "MICKEY's toy!" if she takes one. All of her toys are pink, most of Mickey's are white.

Make sure you spend a lot of alone time with your old dog. Molly is in her crate for a good portion of her days.

Hope all gets better soon!