When I was younger (in the 90s) I read some paperback books about aliens who, if I remember correctly, lived in cereal boxes. They had enemies whose main weakness was a craving for pepper flakes, but who would die if they ate too much.
I believe this is Aliens For Breakfast by Stephanie Spinner and Jonathan Etra. It was published in 1988 and is part of a trilogy including the books Aliens for Breakfast, Aliens for Lunch, and Aliens for Dinner.
This review mentions the aliens coming out of a cereal bowl
When Richard is confronted with a tiny alien named Aric climbing out of his cereal bowl, he doesn't know what to think. But he's even more perplexed and overwhelmed when Aric wants his help to fight the evil Gorf, who is disguised as a popular new kid in Richard's class. What's worse is that there's a very simple way to defeat Gorf, but Aric can't remember it, leaving Richard to fend for himself in the fight against evil with very little but a wisecracking, condescending alien for backup. And if Richard fails, the consequences could be deadly.
One of the reviews for the book mentions the weakness to red pepper flakes
i think i remember liking this book. something about aliens in disguise, and you can kill them with putting red pepper on their pizza