Yes, I think you're right. Rozmawiać is the general continuous action of talking, porozmawiać is having a conversation and finishing it, that's why it s called "perfective". Also this is why porozmawiać cannot be used to describe an action that is taking place right now, as in "they are talking about food".
: ) Hi Mr. Jellei. I'm not certain but I think FakeDiamond might have been trying to clarify why we "need" to talk is being allowed for this sentence? Are the words : "need" "must" and " have" interchangeable if it's an unspoken but implied understanding that this particular "need" will without fail be fulfilled? Like in English, if you used a forceful or exaggerated tone when saying " we NEED to talk! " I guess it could give the understanding that hey, apparently this communication is going to happen whether I want to be a willing party to it or not. (Or maybe this isn't what FakeDiamond had in mind at all....)
Actually 'need to' seems (almost) synonymous to 'have to', at least from the point of view of Polish. Maybe using "potrzebować" + a verb wouldn't be wrong, but it seems unusual to me in Polish, so "Potrzebujemy porozmawiać" is not something I would say. I'd go with "Musimy porozmawiać" anyway.