what is the difference between fe/fo? is there any difference in the alternate constructions?
Fe is used in the south and fo in the north. Any answer should accept using either unless it is a listening one where you should write the one you hear. If you give an answer and it rejects it because you used fe instead of fo or fo instead of fe then report the sentence.
I don't know if there is a guide to it anywhere I just know because I'm native
I don't know which combinations are accepted, but you're not likely to hear "gyda fo" in a possessive construction, as that mixes the southern gyda and the northern fo - but it might be easier for the course creators to accept all combinations, including unlikely ones.