I use like, 4 tactics on Lezalit because I need to increase his int anyway which gives me alot of points. He's usually the guy I play as commander since he has difficulty getting XP in battle by himself since early points are invested into int for him.
Also, prison management only works on the leader, this hasn't changed.
It is quite possible though that having slavers in your party increases your skill, but im not sure how this works or even if it works.
I myself use these:
Bashetur (HA + Looting, Foraging, usually at the end when im actually raising an army)
Borcha (HA + 7 pathfinding, 4 spotting and 4 tracking)
Nizar (Usually knight, sometimes horse archer if I'm joining khergit)
Rolf (Doctor and knight)
Lezalit (Tactics, Trainer and a little foraging)
Matheld (Another knightly companion that I make as strong as possible)
Artimenner (first companion to get a horse + lance, eases up the experience gain for him as I invest quite some early points in int @ engineering, sometimes my secondary doctor since I get alot of points anyway)
Odval (A new companion that can be a horse archer! Yeeeesss!)
Alayen (Usually let him go to be my lord eventually, since I don't like him much but he is a very capable fighter)
SOMETIMES: Only if im not considering raiding villages in the future and feel like im able to stand the sight of them
Edwin (Kind of hate him but hes pretty flexible)
Nadia (Same thing, usually replaces rolf as doctor.)
With my own stats spread around between Pathfinding(6), Wound treatment(7), Surgery(7), First Aid(7), Persuasion(6) and a little leadership(4) and trading(4). And ofcourse 2 inventory management. I get about 21 intelligence as one of my first priorities. I just use my skills to compliment my whole party, as well as having those important skills when someone is down.
For the rest, I use any free points I get in ironflesh and shield since I really dislike getting mashed in one hit. (my build at lvl 30 is about 15-21-21-12, after that I get my str to 30 for pdraw/strike/ironflesh or whatever my build is)