This issue is really strange.
I'm currently making a mini-mod based on Metropolis mod for RTR 6.3. I change the start date, starting armies and provinces. It works fine until I begin to start modding the Roman faction.
I manage to add some provinces and such but when I try to add the gallic provinces of North Italy, the game crashes. I then tried to add the gallic provinces before adding some mediterranean ones, then it worked fine but the game crashed when I later added the mediterranean provinces.
I would be very happy if anyone could help me with this. It almost seems like an invisible province number limit for the Romans.