NOT A TEACHER
(1) I am 99% certain that native speakers would use the singular.
(2) This is true: This morning I did eat a bowl of cereal. It is called
"Crunchy Nut." Delicious. Of course, I used 2% milk. A mature
person ( = old person) like me has to avoid milk with too much fat.
(3) When I go to the supermarket, there are so many cereals ( =
brands & kinds of cereal) that I do not know what to buy.