I keep my companions in 2 groups. Should also note that I turn off companion interaction so I can have all of them if I choose to.
Group 1 I try to keep out of combat. This is Artimenner, Jeremus, Katrin, and usually Borcha just cause he annoys me. Artimenner and Jeremus both take care of medical duties and Engineering. Katrin and Borcha are something like body guards for them and in additon handle things like path finding, tracking, and persuasion (I swear it works better for when I send them as an emissary... Don't know if it really does though). This keeps them on foot, heaviest armor they can wear, shield + sword.
Group 2: Everyone else. Personal guard. Mounted, heraldic mail, sword/shield and if they already have skill in it a bow or throwing weapon. Sole job is to dive into battle with me and hopefully cover my retreat after I take an axe to the face.
That being said, mod I am playing (Litdum) tweeked the AI a bit on spear/polearms and I am liking what I am seeing, I may just toss in a pike/lance.