FWIW, I've raised plenty of fry on microworms alone until they reached a size to start taking fine dry foods like golden pearls and crushed flakes. Guppies are born big enough to eat dry food from the start. Never bred bettas myself, but from what I've heard they can usually take BBS from the start, and anything that can take BBS can take microworms, too.
Also, you could always try boiled egg yolk—it's very nutritious and doesn't involve a messy culture.