Shrimp are out because the BN pleco is a requirement from my 7 year old.
So many possibilities! Just FTR, my bn plecos completely ignored the RC's when they were in with them. It's whatever other fish you put in with them that you'd need to worry about.
My 7 year old has a tank in his room with mollies and ghost catfish. The mollies are out for you, but you might consider the ghost catfish if your tank is heavily planted and not too open. We have five and my son loves to watch them, they school together all facing the same direction, but they hover in the same spot. I've heard they can be delicate in a new tank though, not sure if that's true. Kids also love apple snails, heck, so do adults.
Occasionally female bettas can be aggressive though, so with a bunch you'd be quite lucky if everyone got along, but in that size tank it might not be an issue. They come in so many pretty colors, too. A large school of black neons are also pretty, and a little different than the common neons or cardinals. I hope you'll share pics of it when you are done.