yeah every time a new pictos drops i equip it right away so i can level it up to 4 blue bars, i have every pictos maxxed out and available as a luminary if i want. most are pretty useless but 3-4 are extremely badass and worth using points on for everyone. i didn't really understand the pictos unlock->luminary system until the middle of act 2 either, i'm a dumbass
i'm still in act 2 and haven't figured out who the best chars are, i always roll with the non human dude since he "collects" abilities by killing enemies and i assume he has to be in the party for that to work. i stick with maelle and luna otherwise, since they seem important story wise. i'd swap in the main dude but then its a sausage party. story wise i believe all the characters are there at every fight (except forced splits) so its really just personal preference.
i don't really understand sciel's combat system at all so haven't used her, which probably means she is OP as fuck
another tip for early goers, if you turn in 5 lost gestrals you unlock an ability that lets you loot the stuff under the green roots. can do easily in act 2, and worth the extra 30 mins it will take. All Lost Gestrals are in the open world (or least the first 5, I think the last 4 are act 3).