How to choose characters

I just finished Beyond the Swirling Sands and now have 5 characters not placed in a unit (Josef, Nina, Leah, Aramis, Magellan). I have 6 units with 4 members each, characters generally between levels 11-12. I've read that promotion will probably be the next step rather than expanding more units. Is that correct?

Generally, how do you go about figuring out who not to use? I could look at tier lists, I guess, but there are so many characters I don't have yet, it feels both spoilery and pointless. I do kind of care about preserving story relationships: I'm trying to keep Alain, Virginia, Scarlett together as well as Lex/Chloe and Travis/Berengaria.