Admission of "Mexican" States...
You've got Chihuahua getting statehood at the same time as California, but California had seen significant English speaking migration, and no indication as to any similar immigration in Chihuahua. 90%+ of the state of Taylor are non-english speaking former Mexicans with no interest in owning Slaves *or* voting for members of Congress who will support it.
So at *best* you've got a thin skin of Americans (Miners???) who although Slavery may be legal certainly aren't going to be owning slaves, ruling over a large number of born Mexicans (unable to vote?)
Also, unless "South California" stretches all the way up north of Los Angeles, it isn't going to have enough population for statehood for at least 40 years...