Hey there Leon :)
Yeah.....flakes aren't the best choice as far as food goes for bettas. Its difficult to dole out a proper amount, and often bettas just ignore them all together.....which it appears your girl is doing in favor of the more nummy shrimp pellets. xD
I would suggest a good pellet brand like New Life Spectrum Betta Formula or Omega One Betta Buffet as a staple diet for your little girl, 2-3 pellets per feeding twice a day should do it. For a little variety, as that is of course the key to any good diet, you can also feed thawed frozen bloodworms and brine shrimp to replace a meal as a treat a couple times a week. ;)
Nutrition wise, its probably not the end of the world that she nibble on the shrimp pellets a little.....but the big concern there is over-eating which can lead to bloating and swim bladder issues, which bettas can be prone to.
To help cut down on how much she does/stop her from nibbles on the shrimp pellets I'd suggest waiting about an hour or two after you turn off the lights in the aquarium at night before you drop in the pellets as the corys are nocturnal but your betta is not. If shes still awake(though the hope is that by now shes sleeping), you can distract her with your finger or a mirror or something in one corner and drop the pellets on the other side.
If that doesn't work, you may need to remove her during feeding time. You can simple scoop her up in the cup she came in if you still have it, another small container, or a plastic breeder box works well to and just float her in the tank. Feed her in that, feed the cories, then let her go when most of the shrimp pellets are gone. In time, you can train her to swim into the container/breeder box as she learns to associate it with feeding time. ;)
Hope I could help, and best of luck! :)