It seems to me that you won't end up with a carbon steel weld metal if you run CS on top of nickel. You'll end up with a low alloy weld metal full of Cr, and Ni, and perhaps Mo, or Nb, diluted from the nickel, and depending upon the alloy of choice. This low alloy may form martensite (actually probably will) and require PWHT, whereas the nickel weld metal would not. This would tend to contribute to the possbility of cracking.