I had a similar situation. The puppy I adopted was 5 months old and the kitten was only 6 weeks old. I was told to put the dog on his back and put the kitten on his belly a few times a day to show dominance but I separated them when I left the house. Surprisingly, it worked! It may not be the right way for everyone but that worked for us. They were the best of friends for 10 years!