I've found that most of my younger betta juveniles are able to take live foods such as white worms, blackworms as well frozen adult brine shrimp. I do wait until they are bigger to feed frozen bloodworms as they tend to be harder to chew and I don't want to choke anybody.
Much as people seem to hate them on here, I've found live blackworms as an excellent food for younger bettas. Grindal worms are also good for bigger fry and tend to be bigger than BBS.
Some of my fry are not even a month old, and they will still fight amongst each other. I think older juveniles that have been living separately that are not females are definitely going to show aggression towards each other.