Yes, necro scales with warfare (and intelligence). Increasing necro level will increase your lifesteal.
You can mix hydro + pyro mithout problems.
btw, i tryed both cases carrying lone wolfs:
In my current save i'm in the nameless island with physical dmg caster and a ranged, all is going wonderfull.
The last campaign before this, i did it with a pyro + geo caster and a hydro + aero caster, and more or less went pretty good. Just don't do things like throw fire damage vs fire slugs, venom VS undeads and etc.
