They aren't going to stop being pirates just because they share the same religion. After all, Christian Irishmen were good pirates, weren't they? Now, you won't have the huge "Barbary Coast" piracy, but I think many coastal communities, if given the opportunity, would support piracy. It would more just be called "privateering", at least in a later era.
Perhaps, but would they estabilish the slave trade of europeans captured in raids? Arian or catholic, north africans and europeans are christians ITTL.
I would doubt that, but the slave trade's a good point since North Africa (Maghreb) was historically the main link to the West African slave trade. I can definitely see Christian North Africans doing the same thing Muslim North Africans did and promote the slave trade OTL.
The effects of a Christian North Africa on West Africa demand their own post, though.