Interesting that the comma thing is so common - whenever I proofread things for my partner, splitting immensely long run-on sentences into smaller individual components is always the biggest issue.
As for which to use, definitely ask a supervisor or lecturer beforehand. I've tried both in papers that I've written, and have settled on defaulting to American spellings and style. No one has ever complained, or possibly even noticed it. Given that many university courses will use texts from both American and British authors, I don't think it's much of an issue.