I mean, Artimenner's alright, but he starts high level and engineering can be put on any INT companion. Matheld's also good with her low level and starting stats/skills, but man, Jeremus is a medical machine, and can get another 2 or 3 INT skills maxed in addition to the 3 medical skills, if you can refrain from giving him anything but INT and party skills. Between Jeremus and Borcha, you can get most of the good party skills at a very high level using only 2 companions, freeing other companions for combat or noble training.
You can keep people who hate each other in a party for quite a while and temporarily manage it with the dialogue options when they complain, but eventually one of the offenders will leave the map permanently (though they'll come back eventually with mods like Diplomacy).
Edit: One of the best things about leadership is the wage reduction, 50% at level 10 (I'm not sure if it's multiplicative with garrison wage reduction, though). The party size and morale are also great, and ultimately personal combat skills lose their usefulness over time, which is why I like high charisma and leadership. Something like 9 STR, 12 AGI, 27 CHA, and the rest in INT or STR is probably ideal.