I would disagree that Bettas are best kept alone, providing you choose tank mates VERY carefully.
We have three male Bettas, each living with several Corys/Catfish/Plecs quite happily.
For instance, Brinn lives in his 20 UK gallon tank with 2 Bronze, 2 Albino and 2 Peppered Corys, 2 Upside-Down Cats, 2 BN Plecs and 1 Striped Raphael Cat. Brinn was the last addition to the tank and there were no integration issues to his new surroundings. he plays with the Corys and spends most of his time showing off his finnage to the Plecs and anyone else who will watch.
Slippy lives in a 22 UK gallon tank with 6 Adolfoi and 9 Pygmy Corys, 2 BN Plecs, 1 Striped Raphael, 3 Platys and a Marble Molly. again, the Betta was the last addition to the tank. He was a bit lof an irritable sod to begin with, chasing the Platys around but settled in after a couple days.However, once the Platys and Molly have given birth and the fry removed from the tank, they will be getting themselves part-ex'd against something more to my taste, maybe a couple of Pangasius or Pictus if the tank can cope with them but I will be looking in to that a bit more in depth when the time comes.