Since all undead have [OPPOSED_TO_LIVE], I'd imagine they would still attack every living being they can.
(Necromancers are not counted as "living")
This doesn't seem true anymore. In my old necromancer fort the undead killed everything except other undead, but recently this has not held. E.g there is a thread (and Ive had it happen too) with numerous examples of zombies showing up with elves or humans, living and not necromancers, but allied. It seems Toady rewrote some aspect of undead loyalties and now they are no longer hostile to the living in their own faction.
The problem is that only the zombies that the necromancers bring with them are of the same alliance of the necromancer, all other zombies (Like the zombies of other necromancers or the zombies they raise after getting to your fort) are just the same as the old version.
This makes sense, but has anyone tried to see what happens if a member of your fort does the raising? It might remain friendly...