For safety I think that seperating the animals is the only thing you can do when you are not able to supervise.

When all the animals are together keep Harley on his lead. As the cat enters the room praise Harley for staying in a down position but check him if he attempts to chase the cat. This will take some time to yeild results as Harley is having a ball chasing. He must understand that you will not tolerate it.
What started as a play has been encouraged by the cat running away. The cat had the upper hand as long as it could end the game by turning. As Harley is now feeling even more confident of his authority with the introduction of another animal lower in the pack he is pushing his dominance with the cat as well. You must assert your dominance by not allowing it and making it clear to Harley what is accepted behaviour and what will not be tolerated.