"Will we go together?"
Can one say "pojdziemy" instead of "chodzmy" or are they totally different?
Pójdziemy razem. - it is decided , or you are making that decision right now for everybody. Pójdziemy razem? you are asking someone to go with you
Chodźmy razem, you are suggesting you should go together. Pójdźmy, idźmy
so a question/request versus a suggestion, I think which fits more depends on context.
How is this different to asking 'będziemy iść razem'? Are there notes somewhere on perfective vs imperfective?
I wasn't even sure if "będziemy iść razem" is correct, but the more I think about it, the more I think it is - but it would refer to the process, focus on duration. That you will be walking side by side.
"Pójdziemy" focuses on the result.
Check this post for topics about perfective/imperfective: https://www.duolingo.com/comment/16296174