I don't know if you considered something more predatory, but you could go with a group of 4-6 freshwater Barracuda. They are not aggro and will only eat fish that can fit in there mouth. Keeping them in schools reduces timidity. Stay around 10-12 inches.
And it will add a stark contrast in shape, and are definitely a mid to top water fish.
And it will add a stark contrast in shape, and are definitely a mid to top water fish.