Thanks, Kiara. I love the combination. Willie and Si have all the swimming room they want and the inverts have the bottom to explore and plants and driftwood to climb. Watching the Micros and CPOs is fascinating. Plus, the inverts add almost no bioload.
Leaving the stocking choices of the OP aside, for Australian members, I have found that some of the pseudomugil (blue eye) and rainbow species can work well. This is provided your betta is not overly aggressive or has long fins to be nipped.
I have housed honey blue-eyes, delicate blue-eyes and spotted blue-eyes with a betta male in the past and aside from some nipping, they did fine.
While most rainbows prefer hard, alkaline water and can grow quite large, Mcculloch's rainbows are only a small species and the type I kept preferred a soft water environment. Pygmy rainbowfish may also be another option.