Look, its damage types are different(necro doing physical, rest is magical damage), but they both scale with INT. So you dont want to use necro+aero in same combo on one enemy, as you will attack both of his armors, basically wasting damage.
BUT having more damage types at your disposal is good thing, as you can for example finish damaged enemies with one of their armors down. And necro lifesteal will work with whatever type

So yeah, its viable, just not in the same combo. And realistically speaking, when going for INT character, you want atleast 3 schools (aero+hydro+necro) for example, and if you feel like it take 4th in the mix for diversification (geo being most versatile with spells doing earth+poison and statuses for both physical and magical)