1st option
Give the Knight all the non-magical skills: Identification, Traps, Perception, etc. By level 30, this uber machine will break through everything. And there will be many empty skill slots.
But the main advantage is that the Knight can carry the party very well due to the use of life splitting.
2nd option
4 mages. Moreover, the Sorcerer and the Archer are identical. Then it would be better to have a Paladin instead of the Archer.
I played with the 1st option.

Regarding magic. You approach the Dark/Light guild and click on "learn magic": D And both can learn both magics at the same time.
They can, as far as I know, learn them immediately.