Proposal: Skeptic Exchange
It will have plenty of science answers.
Is it possible to have multiple tags?
Proposals are not organized by tags. A proposal can only belong to one "Category."
Don't think in terms of where the proposal should be categorized. Consider instead where the people who are most likely interested in that proposal are going to hang out. That is the main criteria which defines where a proposal should be categorized… the location of the primary audience.
I would suggest keeping it in Culture since Science is more about objective methodology ("How are telomeres identified in kayrotypes?) whereas “Skepticism” will deal more with epistemological matters where answers will largely be based on ones own presuppositions ("Can David Blaine predict the future"). I'm sure there will be many Scientific questions, just as there are many Math questions in Programming - but the two are still unique.
I don't doubt that discussions of the supernatural will be prevalent within the Skepticism site whereas Science would remain inherently agnostic to those issues since they are, by definition, beyond an objective methodological solution.
I could be wrong here, of course. Most of my conversation at free-thinker gatherings would best be described as a discussion of epistemology, philosophy and a bit of scientific methodology.
As far as I know it is not possible for a proposal to belong to multiple categories.
But I would consider science a better fit than culture, in the end this proposal is about asking and answering scientific questions. The proposal description says this site is for answering questions about pseudoscience, and those questions have definite scientific answers.
If you take a look at the example on-topic questions, every one of them can be answered using basic science and evidence from clinical/scientific studies.