So, basically, I’m using skeletal mages, but it doesn’t matter, any necrotic-damage-based skill will go.
Does spell damage also affects necrotic damage, or I should just grab everything related from passives & skills?
If the skill has the spell tag then it’s affected by spell damage. Hover over the skill and look at the bottom to see what tags it has.
1% spell damage would affect a necrotic spell just as much as 1% necrotic damage. As Travos says, just grab as much modifiers that match the skill tags.
Minion skills, however, are only affected by minion modifiers (% increased minion spell damage, % increased minion necrotic damage), not player modifiers (% increased spell damage, % increased necrotic damage).