I have always done new hatched brine shrimp. Seems to work good for me. I have read that using bbs from the start weeds out the super tiny and unhealthy fry, leaving you with the best and healthest to raise.
I have also been trying some atison's betta starter, along with bbs....my fry realy like it.
my sister uses frozen bbs from the start..thaws it first then tips it in the tank, she has raised 2 spawns this way so far and even though so many people say they will only eat live food, her babys eat whatever falls in the tank lol.