From the principle of measurements, it wouldn't make a difference what material with what coefficients you use. Make sure you fit the measured resistance of the RTD to the temperature curve. This will significantly differ from your Ni sensor, in the same way that TC's of different material has a different transfer function and cannot be mixed without changing the parameters of the conversion-vi. Sorry, practically speaking I do not have experience with Pt sensors in comparison to Ni. Perhaps your supplier can tell you.