I think the best thing to do would be to have the standard NPC's be just normal party members, who can level up their skills and help you, while still having the ability to fight in battles.
Then having hirable NPC's that start as Aides, and cannot be made into heros. These NPC's could be hired like mercenaries, would be specialized with different ranks, and would cost anywhere from a fair amount to LOT (depending on their ranks) both to hire and per week.
You could for example hire a novice navigator, who would give 3 points of pathfinding.
Or a journeyman surgeon, who would give 5 ranks of surgery.
Or perhaps an expert merchant, giving 7 ranks in trading.
Or a master engineer, who would have 9 ranks in engeneering (9 would be the highest, if you wanted 10 you would have to do it yourself, or have one of your real heroes do it.
Others could be:
Hunter (tracking), Drill Instructor(trainer), Tactician(tactics), Scout (spotting), Caravan Master (inventory management), Therapist (wound treatment), Medic (first aid), Diplomat (persuasion), Warden (prisoner management), general (leadership).
As you can see, I have made the leader only skills be party skills too. Perhaps leadership should remain exclusive, but I don't see why you couldn't hire someone to carry your stuff or watch your prisoners (of course if you hired a caravan master you would have to be careful when you got rid of them, lest you lose your stuff).