You can have the necromancer creature and a summon active at the same time, but not multiple summons, AFAIK. I believe the demon is generally considered the best for combat; the ghost can be helpful, but isn't necessarily going to decide to heal you when you want it to. I haven't seen much support for any other summons (haven't tried any of them myself, playing a warrior, though the creature has come in handy).
The official "ULTIMATE MAGE" guide - not exactly what you want to build, but relatively close
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