Druids and Balancers bring a slight problem to the setting. Balancers are worshippers of nature Immortals that can switch over to Druid upon reaching 9th level. Shadow Elf shamans are considered Clerics, however. Paladins and Avengers are the warrior clerics, being one for good and the other for evil. They worship specific Immortals, but they do not gain special abilities beyond their classes.
Immortal introduction is quite simple.
However, their specific worship of an Immortal, unlike the Philosopher's ethos, grants them one or two powers, and perhaps a penalty. Dig out the Wrath of the Immortals book and use the power listed there, or modify and select one or two from the speciality priest listing. However, there needs to be one change. I think that about covers it.