Do followers have favored armor types...light v. heavy? Does it make a difference if you give heavy armor to someone who might prefer light armor? Or is best available armor always the best choice?

Yes to the first two, no to the last one.

Someone who is wearing leather is going to have a better armour rating then if you gave them full Daedric...even if they aren't skilled with it.

I think it's the best available, regardless of type. How the game judges what armor is best, I don't know. I assume it's whatever the base armor rating is.

Followers will choose quality first.

Suppose I could just test it: give one of each type, of similar rating, and see which one they put on. Duh.

Speaking of which, I wanted Adelaisa to wear a set of light Imperial armor, seeing that she's specialized in it and all that. She insists on her set of heavy armor. Seriously, I think I should've considered upgrading it first until I gave it to her.

Still can! Just march her to a blacksmith, take the armor back, upgrade, give back.