I'm inclined to believe that generally better to avoid citing secondary sources as support. Rather the answers should, in my opinion, cite "primary" utilized in those secondary sources (preferably after confirming them) while acknowledging the secondary sources when they main resource used for the answer.
With regards to what constitutes a "primary source" I think a general rule of thumb would be a work which will generally be considered in halachic discussions (or hashkafic works of comparable significance). This is, and probably should be, a loose/subjective standard and no doubt some questions may be best addressed by "sources" which do not fit this criteria. Generally, however, I think that this is descriptive of what most of us are looking for in a source and should be our normative expectation for sources.