I see no problem with just letting your followers be from different npc classes.
If you want magic users use adepts. If you wanna remain close to the original rule, just allow the higher level followers to branch out from the commoner/warrior path.
Also it makes sense that you should be able to attract commoners similar to yourself.
so for a wizard/sorcerer/cleric - Adepts are perfect.
In Traps & Treachery, there is a nice NPC class that isn't to terribly strong, to focus on a rogues thug liek skills. Though the expert also makes a good npc subsitute for skills.