Hmm there indeed too few info about these thing.
The book kept telling us man are bound to these twelve by the birth, but still nothing we can do about it.

The Thm guild worship Thal the ruler of underworld.
Ul'dah also have another temple worship Nald, imbuer of life.
I think Nald'thal probably have the most follower since Ul'dah has huge temples for them.

Con's guild is related to the Nophica

I don't know what's arcanist <3