It will depend on tank size and the temperment of the Betta you are looking to try out with them. But Galaxy Rasboras or Celestial Pearl Danios are known for being shy, unobtrusive fish, their relatives do well with male Bettas in community tanks. I would do 6 of these guys, one male Betta and perhaps a trio of Oto cats in a 10 or 20 gallon tank with live plants and a lot of filtration. If it's heavily planted I think you'll have less issues with mixing fish, and of course, put your Betta fish in last so he doesnt see the whole tank as his before you add his tank mates. Add the Danios 3 at a time after a fishless cycle and then the otos (after quarantine! They are very delicate in the beginning, but once they are established, they are generally very hardy) and last the Betta. Each about 4 weeks apart. If you already have the Betta, I would move him temporarily to a smaller tank while you get the bigger tank cycled and add the other fish. Then put him back.