You may find, like we did, that bettas can be very fussy about what they eat, and will not touch food such as flake when it is put in for other fish. Our first betta Floyd wouldn't eat anything but fresh bloodworm, so we had no problems, he just ignored all other food until his was put in. Your cherry barbs will probably eat anything given to them and wont get sick by eating food meant for the betta. Likewise, if the betta is not fussy, it shouldn't have any effect on him either.