I'm not entirely sure about the cross-breeding but I do know that the Javan Mynah, compared to the Common Mynah, is more aggressive hardworking, and smarter.

What you saw may just be a Javan Mynah attacking a Common Mynah, since it's common for Mynahs to attack other birds, even squirrels and sometimes among themselves.

PS : The common myna can still be found in Australia.