I Want to know the difference between display managers and found this question, but it does not include SDDM. Should I edit that question to include SDDM or ask as a separate one?
IMO you should ask a new question. That one is old and very specific.
You should write the question to include comparisons of ALL current display managers, then we would have a good and detailed up to date comparison.
Also, a lot has change, so the current answer is likely dated anyway.
As this would be a very useful question, I would be willing to give a bounty to anyone who wants to write a grate answer for us ;)
You shouldn't do either. Big-list type questions aren't well-suited to the Stack Exchange format. They're a pain to keep updated. New entries don't get anywhere near as many views as older ones, so quality control on them is more difficult.
Also, other big list questions (like What Application Indicators are available?) have this notice on them:
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. While you are encouraged to help maintain its answers, please understand that "big list" questions are not generally allowed on Ask Ubuntu and will be closed per the help center.
Also, the question that you asked is also soliciting opinions:
Why should I choose one over others?
I'd VTC just for that, even if it wasn't too broad.