EHC,
Only use custom if the type of RTD you have is not listed, i.e. Pt 3750 or Pt 3960. These predefined entries are just that, predefined coefficients. If you select custom, then you definitely need to find the A, B, and C coefficients. All of this could probably be found in the vendor's RTD documentation, web site, etc. Regarding excitation, again, what does the documentation specify? The 9217 can supply 1 mA, and can be turned off if needed.
Operating Instructions and Specifications - NI 9217
http://www.ni.com/pdf/manuals/374187c.pdf
Making an RTD or Thermistor Measurement in LabVIEW
http://www.ni.com/white-paper/14353/en/