otos... I recommend to get as many as you can... space permitting.
Apple snails are also OK.
However if your tank is too small or uncycled, stocking with tank mates isn't usually a good idea.water quality issue can lead to uncomfortable fish and problems with deterioating finnage.
Otos tend to be finicky upon purchase, some will die, but those that survive tend to last.
They should be supplemented with blanched veggies as many do not know how to eat algae wafers.
Mystery snails/apple snails will readily take fish food in addition to scavenging for food.
Personally I think otos are much much cuter.
back to the companion ship problem, Bettas don't need or want someone to share their space, often times I add "companions" when I feel the tank is getting a little boring with just one betta. lol. But my betta couldn't care less. that's just his personality...