Praecox great choice for your size tank. Aggression not really a problem with rainbows, it’s all show and display mainly.
But I would stick to smaller ones like praecox, parva and bosmani etc. Some rainbow can get quite large and all are aggressive fast feeders, during feeding more timid species like tetra won’t stand a chance at getting food with bunch of large aggressive feeders whirling about, they’ll shy away into weeds and unless you target feed them they won’t get any food. You’ll be lucky if a scrap of food makes it 4” below water surface.