I already have a schooling fish tank, a 30 gal with neon tetras/harlequin raspboras/rummynose tetras, so I was trying to go a different route here. But it doesn't look like that's going to work out.
I am definitely doing a black background. Substrate is Flourite, so it's brown.
I think I've already settled it. Going to start by adding 3 or 4 more bosemani rainbows to round out a school of 5-6. Then I'm going to give it some time and evaluate how crowded it looks. If I feel comfortable with it, I'll take Kymmie's suggestion and add a small school of Congo Tetras.
Thanks for the suggestions guys!