It depends on the bettas temperament and the other fishes as well.
I would not put neon tetras with bettas since neons are known to be nippy.
Things with long flowing fins and bright colors are not usually good things to put with bettas (such as guppies). This doesn't mean it's impossible but it's best to be cautious.
If you must put something with your betta (only if he is calm enough for tank mates) I would go with pygmaeus, hastatus, or habrosus cories.
They should be kept in groups of 6 of their own kind. I'd go for habrosus over the others as they tend to stay on the bottom more while the other two will go all over the place.
Be sure to compare other fishes temperature needs as well since the bettas minimum temperature is 78F.
As Mamashack said, they don't need company. I actually prefer mine to be alone now after an unfortunate incident having mine with tank mates. It of course is not something that is guaranteed to happen but something that could happen and that you should keep an eye out for.
Mine was with sterbai cories and mystery snails in a 20g long and would gorge himself on their food and eventually he got pineconed.
Thankfully I noticed quickly enough (apparently) and had the amazing help of WildBetta and got some epsom salts in ASAP and he has recovered and is now in his own 10g.
HERE is a caresheet that is a good read, if you havn't looked at it already.