I'm very early into the game.
My first idea was to multiclass all the companions in order to test the different possibilities.
Eder: fighter+rogue is ok, i like the idea of having a tank that can also do some decent damage outburst.
(done some tests with the console command to level 19, and i found it better than pure fighter)
Aloth: wizard+fighter, in PoE1 i was using him as a fighter/tank with the "Concelhaut's Parasitic Staff" + "mirror image" + "iron skin", he was very effective, my idea is to do the same here.
Xoti: pure priest, i feel like you need to spend lot of point in both priest and monk classes, and so i've gone full priest with her, also her stats are very average for a warrior classe.