/Dark is a great thematic secondary for Necro, I've got a Necro/Dark around level 30 and he's very safe, I skipped Petrifying Gaze and Black Hole.
Cold is a good secondary overall, and since starting at 26 your pets can heal themselves, I could see this being a strong combination. You could slot Life Drain as a heal instead of an attack for your own survivability.
I wouldn't discount the attacks on Necro. They each have a chance of spawning an uncontrollable pet, so you can get 3 extra pets out of taking the 3 attacks.