I feed mine wafers and frozen food, too. I found that if I feed very small food, like baby brine shrimp or daphnia, they snuffle around and find it in the sand, while the betta can't seem to gobble them up before the cories get a chance. It's kind of funny to watch the betta during the cory feeding frenzy, darting around like "wait! What are you guys eating?? I see you eating but I don't see food!!" Sometimes if I feed them something bigger my betta swoops in and gobbles it before the cories even get a sniff, then he ends up way overstuffed.
I loooooooove my cories, I think they're so cute.