The other aspect is the size of the tank. The only other fish suitable for the tank size are very small fish and tiny fish are likely to trigger a "FOOD!!" response from a betta. Even if he couldn't catch them, the small fish would know they are sharing a tank with a predator which would stress them. In a larger tank this would be less of a problem as it would allow larger tank mates.
I like a heavily planted tank, especially in a smaller tank since it helps so much with water quality and I just love the look, but all of the fish mentioned appreciate a well planted tank too.
What's the GH of your water? If you're in a hardwater area, I'd go for nano livebearers like:
(male only to prevent getting overstocked with fry - and the males are prettier anyway!)
. Mini rainbow fish that thrive in harder water
Nano fish would definitely be what I'd suggest for a ten gallon! A colony of male only Endlers? Super pretty, colourful, active and not difficult to keep. Pseudomugli are tiny but flashy, gorgeous, active and fun to watch, and a gang would be fine in a 10g. Or perhaps celestial pearls or galaxy rasbora.
If the water is soft, then you have even more options! Go on youtube and search for nano fish species, there are so many, and the tank becomes like a mini world, without a big bioload. Look up Badis, Chili rasbora, ember tetra, green neons, kubatoi rasbora, there are many more as well
10gs is a small tank in terms of a mixed community, but you can do a lot with it if you think of it as a nano tank, prepare it right, and can have those tiny fish in the kinds of numbers they need, without overwhelming the tank with a high bioload, especially a well planted tank.
10g is too small for the larger, popular community tetra like neons and cardinals, in my opinion. But you could have 12 plus green tetra or celestial pearls in a well planted 10g without problem!
Or a dwarf corydoras tank! I'm biased since I love my pygmy cories, but also want to keep hasbrosus at some point, get a colony of those breeding, and my pygmies have been doing great in a 15.5g for a few years now.