Valerie: As soon as you can, replace her tower shield with a heavy shield. She takes a -2 to hit with the former until level 5 and she has enough trouble hitting things as it is with her mediocre strength. Give her tower shield back then. Full plate for her should perhaps be your first big purchase.

Amiri: Don't stress if she seems to do a lot of damage but goes down frequently. That's what barbarians tend to do. I suggest you don't overreact by spending her feats to get single points of AC. Later on you can use spells like blur and displacement and her own DR to keep her up. She can only use one stance rage power at a time so only take one. Lethal stance doesn't stack with Linzi's inspire courage so if you want both companions in your group take reckless stance instead. Enlarge person is fantastic on her given her oversized bastard sword. Enlarge increases the die of damage as well as str and reach. Rage is toggled. She can't rage if she's fatigued.

Linzi: Sing, sing a lot. But don't forget she can do other things while singing. She might look like a decent archer but she's really not with only 8 str. I suggest concentrating on her spells/buffing/skills. She may be your go to for thievery very early on but she will quickly be supplanted by Octavia there who has skill focus. Hideous laughter is great on humanoids. Heroism (a 2nd lvl spell) is an amazing long duration buff at early levels.

Harrim: Channelling positive energy in combat heals your opponents in range unless you have selective channel which requires a 13 cha (which he doesn't have). Destructive smite is toggled. Cast bless before combat when you spot an enemy. Lesser restoration (which clerics gets at lvl 3) cures fatigue and ability damage. Resting should heal a little bit of the latter as well. Remove blindness comes at lvl 5.

Jaethal: Positive energy hurts her. Negative energy heals her. Unless you have a negative energy channelling main, she will pretty much use all her spell slots healing herself. I suggest waiting for enemies to engage on other characters before sending her in to clean up.