The Wizard class should show up in the leader customization screen and should have access to all costumes for your wizard. The idea of the class is to choose a sphere of magic as the specialization and then choose two No Specialization placeholders. The spheres are all mutually exclusive, so you can only choose one.

However, the way the class was created, I cannot forbid the Eternal Lords' alignment specializations from being chosen. The class is not designed to have them chosen in addition to the sphere.

As for your Leader's development while leveling, once the game starts, you can choose your corresponding sphere of magic upgrade. This will remove the other sphere upgrades and allow you access to upgrades that are only accessible to whatever type of wizard you chose. You can, however, have your leader specialize in one Sphere of Magic, but have hero upgrades in another due to the setup.

Note that the choice of wizard upgrade for your leader does not affect the research options you will have.

Firstly, you must have both Golden Realms and Eternal Lords expansions for the mod to work correctly (as noted). It might work without them in some cases, but the Death Sphere will likely be problematic as it uses some of the necromancer spells directly.