Our early experience with winding pitch issues and CAT generators was with the oil drilling industry, the SRSE and SRCR generators were 5/6 pitch to provide better service with SCR drives found on the drill rigs. As CAT expanded into building service generators, mostly standbys and some islanded power, the 5/6 and 6/7 pitch machines produced did a pretty good job, probably more from being way overbuilt for their kVA rating. CAT moved to 2/3 pitch for one primary issue, it is what the competition had and what customers requested, now most of our generators are 2/3 pitch.
Paralleling machines of different pitches can cause circulating currents, as pointed out above, depending on load factor may or may not be a real issue. The actual problem is heating of the stator core, and resulting deterioration of the winding insulation. Many times you can feel localized hot spots on the stator that don't always show up on the stator RTD's (if there any even installed).
I have always been told that neutral resistors are to limit fault current and usually have no effect in solving circulating currents caused by pitch differences, we usually use grounding reactors properly sized to mitigate circulating currents caused by winding pitch difference.
Here is a link to a company we use quite a bit to help us with these issues,
Also at one time ASCO had a white paper discussing neutral grounding reactors, and how to handle generator neutral grounding, with some comments regarding winding pitch differences.
Our experience is based on units from about 200 kW to about 4.0 MW, mostly CAT with some KATO and AVG. We have yet to find a "standard" solution for this issue as we always seem to be doing the system analysis on a case by case basis.
The EGSA Handbook also has some information on this issue from a general standpoint that you may find helpful. Their website is
Hope that helps.