There are tons of things you can do. First, IMO, the tiger barbs suggested will pick the betta's fins completely. Tigers are notorious fin nippers and limit what else you ca have safely.
The redtail shark could cause problems down the line once fully grown. I would just make sure to keep a careful eye on things as it grows.
I would start at the top level, the betta and maybe get a dwarf gourami or two. You may also consider a top water minnow or killie. A few contrasting color Sail Fin Mollys would work as well.
In the middle, a nice school of Head and Tail Light tetras along with a school of regulor and albino cherry barbs would look nice. You could also get a nice school of penguin tetras. Penguins are one of my favorites. You could also add a small school of gold barbs or harliquin rasboras.
On the bottom you could go with shrimp or corys or both. You may want to look at the moderate size loaches, but keep in mind many loaches grow large eventually. If shrimp are out 2 pairs of kribs would look nice. I would mix albino and normal kribs. Rainbow Cichlids would work also.
