The earliest I can see it resembling OTL's Civil War - that is, primarily North-South - would be 1850, over the admission of California and/or other Western territories as free or slave States. Taylor would not be fighting for the rebels; despite being a slaveowner himself from Louisiana, his hatred of secessionists is well documented.
However, Robert E. Lee hated secessionist and thought they were going against the ideals of the Founding Fathers but he still fought for the secessionist South in the Civil War in OTL.
Joe Johnston was against secession for the same reason as Bobby Lee but he fought for the South.
Jubal Early was one of the most vocal opponents to secession in the South before the war but fought for the South during the War and becames its most out spoken supporter.
John C. Breckenrigde, Jefferson Davis, P.G.T. Beauregard, John B. Gordon and Zachery Taylor's own son Richard Taylor all opposed secession prior to the War but all served the Confederacy.
It was a matter of each of them believing that their duty lay more with family, friends and native States than with the Union.
Was Zachery Taylor so different from his son that he would not have done the same if the same situation occured in his lifetime?