Aloha guys,
I wanted to add that my long fin Zebra Danios are hyperactive little athletes as well. They are fast and agile little things that I bought a small shoal of 6 of them to to cycle my 30g long... They shoot around at lightning speed changing directions twice in the blink of an eye! Super cool to watch...
I've seen them kept in small tanks and they are extremely hardy little buggers, they can SURVIVE in all kinds of less-than-ideal circumstances (I.e. Small numbers and tiny tanks... I even saw at an LFS a single one living with 2 GIANT Oscars for more than 7 months before he got relocated)
But can imagine them worrying a Betta (who tend to swim SLOWLY as they drag those bed sheets for fins around) I know my little school of Danios were worrying the balloon Mollies I Added to the 30g long tank after it had cycled.
I keep Triops with my Betta to clean the tank bottom. I have also heard they get along well with peaceful bottom dwellers like Kuhli Loaches and Cory cats.
Good luck with yours

cheers
LadyDragon