Au vs Pour?
So I've been Googling for an answer, but all it comes up with is A vs De, which I don't think will help.
What is the difference between the two? Can they be used interchangeably? (For example, Duolingo marked me wrong for using pour before petit-dejeuner).
I'm a bit confused on the specifics of your question, but even so I can tell you that there aren't really any one-to-one relationships between English and French use of prepositions. And like in English, they can often seem arbitrary. This is the case with most languages. Short answer: You just have to memorize them.
Also, petit déjeuner is not hyphenated and is always preceded by the definite article le (as it is a masculine noun). This may go some way to explaining why au (à + le) is used and not pour.
ex Je te verra au petit déjeuner → "I'll see you at breakfast".
But, as aistobe has pointed out, prepositions do not align between French and English so need to be memorised.