I prefer to feed once a day too - it just works out to be more convenient for me.
As for amounts, bettas are prone to bloat and constipation if they eat too much - and they ARE greedy little pigs. I only feed my betta about 5 pellets a day - about half what you are feeding.
I think that the amount you are feeding the corys is fine so long as you keep the gravel clean. Dirty gravel can lead to eroded mouth barbels.
Both fish go nuts over partially cooked, shelled peas. Once a week, my betta and bottom dwellers get some veggies instead of their usual packaged foods.
My betta is weird in that he won't even touch frozen bloodworms - while the corys can't seem to scarf them down fast enough