I'm going to feed them crumbled-up fish food
This just wont work, Baby Bettas wont eat it they
must have live food
For the first three days after becoming free-swimming the fry will need very miniscule foods. Infusoria and tiny free-living nematodes like Vinegar Eels, Microworms, Banana Worms, and Walter Worms make great first foods.
After 3 days they will be large enough to also start feeding on baby brine shrimp.
After a week continue to feed baby brine shrimp, but feeding the tiny nematodes will no longer offer any nutritional value.
At 3-4 weeks continue to feed baby brine shrimp, but you can also introduce finely grated frozen foods. Frozen Bloodworms and frozen Daphnia work great. The Hikari brand is a good choice because their frozen foods have vitamins added before packaging and the food goes through a strict parasite decontamination process.
At 4-5 weeks you can keep feeding brine shrimp, but you can also introduce live Blackworms. These are aquatic worms and are great because they will live in the tank until they are eaten. Grindal Worms are also a good food that can be introduced at this time. Brine Shrimp, Blackworms, and Grindal Worms can all be fed to adult bettas as well.
At about 8-9 weeks the fry are starting to mature. Live Brine Shrimp and Bloodworms are still ideal foods, along with frozen foods. At this time dry foods can also be introduced.
And as for the fry, I have a 55-gallon I can divide up for them
Thats not good, Once the fry are old enough they will need to go into their own tank.
This is the correct way to house male Betta fry.
Do you know what age baby bettas will start fighting each other if they are males?
Around 8 weeks.
Do you know anything about Betta genetics?
Look dont take this the right way but I do not think you are ready to breed Bettas