danios aren't a good idea in only a 10 gallon. IMO, most fast schoolers need to be in groups of 5 at the least with 20 gallons as to avoid stressing each other and your betta. of course every betta is different, but a tank full of schooling fish who need more room would be to stressful for most bettas.
there are fish like platies, mollies, cory cats, etc who aren't as crazy active, but still school and look pretty.
Most Mollies will get too big for a 10 gallon though.
Cories usually get along pretty well with bettas. You could put four or five regular cories or six+ pygmy/dwarf cories. Platys are also usually good, but they might need something larger than a 10 gallon, I'm not sure.