A quick meta question, if I have a question that relates to another question, as this one does, do I post it as a comment? If not, how do I reference this question in my question? Thanks.
If you have a question that is related to another question, then post your question as a new question. You can then link to the related question and possibly say why they are related. You can simple paste the url of the related question into your question. It will automatically be formatted. For example
If your question is very similar, you should explain how your question is different (otherwise it might be closed as a duplicate.
You should never ask a new question in the comments.
Comments can be posted on Questions (and Answers) seeking a clarification or pointing out apparent typos, etc. These should be "shallow" questions, in the sense that you expect the original author of the post would be able to address gaps in their post (or make a quick reply comment) that resolves the issue. [Often once the post itself is suitably edited, e.g. to cover a missing case, the Comments below it may be self-deleted by their respective contributors.]
More substantive problems should be posted as new Questions, with a link to the original helping to supply context. As a rule of thumb I would ask myself whether the question being raised makes sense mathematically as a problem in itself, however much overlapping there might be, or if the question is really dependent on the formatting and phrasing of the earlier post (so not of independent mathematical interest).