Could Clerics join the Red skulls? Because they technically "worship" The way and there "Saint Bresley."

For example: I am in a particular predicament in that Conika is a cleric of a God of Pacifism and nature, and thus wishes to preserve life. But if she joins the red skulls, she has to admit that everyone around her is dead.

Would I get spell failure if I joined?

Find out in game. The Red Skulls are likely to embrace anyone into their organization.

The current members aren't entirely sure at the moment, they never had a priest.

Get in touch with a NPC Red Skull then. =)