Bettas mouths are not really made to handle flakes easily but they will try. It is just easier for them to grab a pellet and most are not going to work that hard to try with the flake food and will ignore it unless they are REALLY hungry. It is easier for me to feed them pellets but to give you a heads up, some do not like the pellets either and there are some pellets that are supposedly made for bettas that are too big for them to get in their mouths and it does not work to break them up as this makes them sink too fast. The only really successful ones that I have found for mine are the Ocean Nutrition Atisons Betta Pro and Atisons Betta Formula and the Hikari Betta Bio-gold pellets.
But for the little ones who do not like pellets you will not be able to make them eat them. Bettas are picky eaters and will only eat what they want to eat. You cannot coax them to eat them or beg them to do it. If they say no that is it. I have never known a betta to refuse bloodworms though so if you get one who will not eat anything else, try them. Shrimp does not go over well after they are grown and it is messy in your tank. It is also too rich for an adult and should not be more than an occasional treat (no more than 2 times a week) for even those bettas who will eat it.
You can go nearly bankrupt trying to find a food for a finicky betta so it would be my recommendation not to try to buy a great number of foods right away. Try to find a couple of foods that they really like and later find a "treat" type food that you can supplement their diet with but get it in a small amount as most foods really should be replaced after being opened for 6 months and he will not eat that much of a large container.
Rose