If you're adding extra civs then you're probably not going to have much joy. I've never tried it myself, but I've heard that other people have had trouble.
Governments should cause no crashes.
Units only crash (for me anyway) if the new unit has a folder or .ini file with a spelling mistake.
I've added extra improvements and techs without crashes either.
Your best bet would be to go back to the original rules and change one thing at a time, this way you'll see exactly what change is making the game crash.