Pam, I think if you suspect the food and are concerned about similar problems in the future, then it's probably a good idea to mix two types of food. That way if something seems off in brand 1, the rabbits can still eat brand 2 without having their tummies upset by a sudden food switch.
I'd be willing to bet that a lot of the mycotoxin problems are related to how stuff is stored. If it's really humid, or sitting directly on the floor instead of on pallets, or kept in an air tight container, it would probably encourage mold growth even if it looks good to the naked human eye. But there's no way to know how the stuff that you buy gets transported and stored by the middle men before it gets to your feed store.