From what they said in the update 41.
Water + Necro = Blood Rain
Source Water + Source Necro = Blood Storm
Fire + Necro = Corpse Explosion
Source Fire + Source Necro = Mass Corpse Explosion
The spell used has no effect on the combinaison it is the type that is important, so I think Earth + Scoundrel would be the ambush spiders or something.
Or maybe you can reuse crafted spell but they won't give more spells than a basic elemental spell would.
And yeah I just watched the update 41 and one of the spell they use for necromancy skill crafting is a necromancy summon. So the Necromancer has his own summons but will still need to invest in Summoning to buff them.
