You tank is 23 x 16 x 11 which gives you more options than a taller than long aquarium. Your GPH is 60. You can plug in all the particulars here: AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor
. Keep in mind the site is somewhat conservative. And choose "User Defined" for filter and dimensions.
I always have recommended a minimum shoal size of six. However, when I asked Rachel O'Leary, aka msjinkzed, about shoal size when housing with a Betta she said at least eight but 10+ is better; especially mid-tank and top dwellers. So that is what I would go with.
When planning residents divide by upper (Betta), middle (Neons, Rasbora) and bottom dwellers. You have the first two covered so you can add bottom dwellers. If you want to continue with smaller fish, Habrosus Cory are good. While Pygmy (one of my favorites) and Hastatus are also dwarf Cory they are mid-tank dwellers.
Plugging in the Aqadvisor, the long and short of it:
10 Neons or 10 Pygmy Cory (Neons can be nippy if you get a long-finned Betta)
8 Panda Cory
If it is well/heavily-planted, you can add shrimp. But wait at least a couple of months for the tank to mature before you do. Here is this Forum's cycling tutorial to help you make your tank safe for all.
Begin with just the Betta and add the others after the tank is planted (live or silk) and cycled. To avoid issues, float them in a dark tank and release. Leave the lights off for at least an hour to give the newbies a chance to scope out the tank. Members of this Forum highly recommend using SeaChem Prime as your conditioner and I recommend SeaChem Stability to help cycle and to use when adding new residents.