There are two (2) kinds of cylinders, one which has a "straight" profile, the other has a "step" or a "shoulder" about mid-way in its length. Are these two types interchangeable?
Yes, the Pietta Colt 1851 .44 cylinders and the Pietta Colt 1860 .44 cylinders are interchangeable, as are both the rebated and non-rebated .44 replacement cylinders for each.
Even the barrels, cylinders and grip frames for some of the Pietta Colt .36's and .44's can be swapped, although that's not to guarantee that some fitting won't be required.