Plus, the Sunni are a minority compared to the Shi'ites, whereas there are a good amounts of Catholics, Protestants, and Orthodox. There are more Christians than any other relgion.
This may seem rather insignificant, but actually, there are more Sunni muslims than Shia. In fact, taken as a whole, Sunni Islam is the world's second largest religion. Shia are just the majority in Iraq and Iran (and a couple other middle eastern nations), and they are the ones that dominate the news, sending the false impression that they represent the whole middle eastern world. And though there are more Christians than any other religion, they are, perhaps, the most divided of them all. There are far fewer sects of Islam.
And as for the mod pack, here is my proposal: Christianity, Islam, Buddhism, Paganism, Hinduism, Atheistic, and perhaps Roman Catholic. Rather than Catholicism being a separate entity, it could be replaced with another Pagan-type religion, but I am not familiar enough with Paganism to give any suggestions.
These may not be the all of the most dominant, or "important" world religions, but I believe these are best representative of the civilizations at hand. Note Judaism, an important religion in terms of its impact (spawning the world's two largest religions, Christianity and Islam) was excluded simply because it is not a dominant religion in any of the provided civilizations.
There are other flaws of course, but I just look to the leader of the particular nation to determine the religion. Rome, for example, started pagan, but by its collapse had embraced Christianity. However, during the reign of Julius Caesar, Christianity didn't even exist, thus I would label it pagan. Same with Mao of the Chinese. There is no doubt Buddhism and Christianity had their respective impacts on those civilizations, but it breeds too much controversy to go into which has had a more perservering impact and which is more important in today's terms.