I think there is ambiguity stemming from that linked post. It could be interpreted as asserting that all questions here should fall into exactly one of three categories: research-level, popular-science, and homework.
My interpretation however is that this is not the case. There are non-homework questions that seek technical answers (rather than "please explain the universe without math") but could be answered by just about any practicing physicist who puts some thought into it, and thus are not restricted to those with advanced knowledge in one specific subfield.
For example, the two questions I have asked on this site1 fall into this category. They should not be tagged with any of those three tags (and I will rollback any edits that attempt to do so).
This has the benefit of allowing different coarseness of filtering. You can choose to only look at research-level, or you could choose to simply block popular-science and homework so that you see both domain-specific and generic, not-too-simple physics questions.
In conclusion: Feel free to use research-level if you want to restrict answers to the level you would get if you asked the question of a specialist in the field. You yourself don't need to be publishing papers on the topic. After all, this sort of tag is more for the level of answers you want than the level of the question. (If the question were at the very cutting edge of research, why then by definition no one would have an answer, and moreover anyone with the answer would publish a paper before posting to this site.) If you don't use the tag, no one is forcing your question to be relegated to popular science.
1 Neutrinos vs. Photons: Who wins the race across the galaxy? and Mars just collided with Earth! A question of eccentricity.