The ladies are correct that the fish compatability is a huge issue with your tank.
(Good job ladies!

)
Bala sharks get large enough to need a 75-100g tank by the time they are adults.
Bettas should be observed for temperment if attempting a community setting.
Betta Splendens (specific species of betta that 99% of the pet stores/fish stores carry) should not be kept together in 99.999%$ tanks unless it is females that were raised in the same tank, and even then should be watched for temperment.
Again, the ladies have this covered so I am adding a back-up statement to echo what they've said.
