You're correct, you'll need Electric spellpower and Light spellpower. Note that equipment boosts like "Radiance" affect "light and
alignment" spellpower, which includes Evil.
In most cases, Universal Spell Power stacks. As a general rule of thumb, if it's in an enhancement tree, it stacks. However, Potency (eg, on
Pansophic Circlet)
doesn't stack. If you have a Potency and a Magnetism item, the game will use whatever bonus is largest - that's typically Magnetism if the item levels are comparable because Potency scales a little slower. That being said, Potency is still useful for casting breadth, especially for a class like Wizard or Alchemist who has access to a lot of different elements