From what it looks like, I'll probably try different polearms ad see how they do on horseback. I was staying away from polearms mostly because I also enjoy small scale battles as well (such as bandit hideouts) and in my experience they seem to do better with shorter weapons in those and I didn't feel like remembering to swap weapons.
To the switching weapons problem:
What you can do (also in hideouts), is giving your companions 1 weapon for reach (bow or 2 hand weapons) and 1 weapon that is 1 handed with shield.
So when you order them into shieldwall formation, they will use shorter weapons for sure, they can also charge like that.
When you let the companions choose, they tend to take the weapon with more damage or reach i think (bow/ spear/ 2 handed weapons) .
But with "shieldwall" or "hold fire" command, you can force them to use shorter weapons in close quarter combat, because they switch to the shield or their "side weapons"