fcmf replied as I was typing but I'll leave what I had typed even though it repeats a lot of what she said
As the others have said, very small fish like salt & pepper cores and celestial pearl danios (aka galaxy rasboras) need a lot more than the usual 6 for shoaling fish. I would say 10 is the minimum number for these fish.
There is also the problem that celestial pearl danios need water that is harder than yours. Your hardness is 32.5 ppm and they need 90 to 268 ppm. But there are other small fish that would love your water such as ember tetras and
Microdevario kubotai (sold as neon green rasboras or green tetras). And with your hardness, any of the tiny fish in the genus
Boraras would be fine.
You might have trouble finding salt & pepper cories, Corydoras habrosus. Not many shops stock them. If you can't find any, pygmy cories would work just as well, again at least 10 of them. (Make sure you don't buy peppered cories, Corydoras paleatus, as they grow too big)
I agree with Matt. Bettas are not community fish, and not with fish like tetras that have the potential to nip their fins.
Some suggestions for you. I know this makes the community creator over 100% but with small fish like this not having enough of them makes them stressed, and stressed fish have a bigger bioload then non-stressed fish. So with small shoaling fish you can push the stocking level.
Option 1
10 x salt & pepper or pygmy cories
8 x neon tetras - or 10 x green neon tetras if you can find any
10 ember tetras or one of the Boraras species.
Shrimps to eat algae
Option 2
10 x salt & pepper/pygmy cories
8 neon tetras or 10 ember tetras or 10 Boraras
Pair honey gouramis (instead of a betta)
shrimps to eat algae
Green neon tetras are slightly smaller than neons, and despite their name they have a blue stripe which has a greenish tint when looked at from above.
Honey gouramis are fine as a male/female pair. Natural coloured honeys (the colour in the photos in the profiles on here) and the yellow variety are easy to sex; the red coloured honeys are more difficult, and you don't want to end up with 2 males.