Welcome to TPT!
Nice start on the tank, very nice work with that cave.
Unfortunately, I think the lovely purple plant you have is also known as "Purple Hedge" - which isn't an aquatic plant and would do much better in a pot as a houseplant.
If you want a plant in that spot that might do OK in low lighting but still have some red or purple, you could consider Sunset Hygro, Crypts, or Ludwigia repens.
I can't tell but the green stem behind it looks like it might be Bacopa, if so that should do fine.
I wouldn't mix Tiger Barbs with a Betta, the Betta will end up nipped and constantly harassed as they won't be able to resist his fins. I also wouldn't try CRS for the reason Seds mentioned; you might try RCS once the tank is fully planted out as they are 1) cheaper, should they turn into just another Betta snack, and 2) would have more places to hide. Or Amano shrimp should work; they're large enough to be safe. Hillstream loaches need cool water, lots of veggie matter, and fast currents- so IMO have opposite needs from Bettas, so unfortunately another fish that I wouln't put in there.
You might consider some Sidthimunki loaches, and I think dwarf rasboras also might work nicely, or a school of small tetras. Check out the fish that www.InvertzFactory.com
carries, they specialize a bit in "nano fish" and have quite a few species that would work well in your setup.
Last but not least, it can work best to not use the flash on your camera and to take pictures in a completely dark room with only the tank lights on (I usually take pics at night and turn off all the other lights in the room).
Keep us posted on how this tank comes together; I love watching this process!
The plant on the left is Anacharis, which should also work.