I've been messing about with pedhealth.xml but I don't know how the three traits (strong, average, weak) are distributed. Do different types of peds (citizens, cops, swat, etc) have corresponding traits or is it just randomized?

Sorry to respond so late, but when does it crash? If it's while loading, try to remove EnableMPVehicles (not entirely sure that's the exact name - it's close though). That's what fixed my crashing issue.