past publications

If one of your stories appeared in a "decent" publication, I don't think the fact that it was ten years ago matters a lot. Do you feel obligated to list publication dates for all your work? Some agents say they want those details but others don't seem to care. Check online and see what the agency expects in a query letter.

Depending on what kind of nonfiction you have published (letters to the editor don't count), I would definitely mention that fact, along with awards you have won.

On the other hand, if you think your credits don't look impressive enough, it's possible to come up with some kind of all-inclusive but vague statement like "I have published fiction and nonfiction in a number of magazines, including [the "decent" one], and have won several awards for my writing."

Ultimately, an agent's reaction to your query will be based more on the work you are planning to submit than on your credits.

I would list both. It sounds like your journalism work is quite impressive. The two fiction credits show you are versatile.

Everyone will have their own answer, but...

I only list publications that are on a roughly equal reputation level to the one I'm submitting to. So for a pro market, I'd only list pro and established semi-pro sales (I choose the best three). When I only had sales to small token-payers, I'd list them on subs to other tokens, but not to pro markets.

I wouldn't list non-fiction for a short story cover letter.

I haven't found it makes any real difference. There was a point where I got better and started getting through to later rounds and acceptances... but it happened at a bunch of markets at once, so they all had my no-credits cover letter. My writing improvements made more of an impact than my credits improvements (though one led to the other).

I don't list nonfiction. Being able to write nonfiction well has no bearing at all on being able to write nonfiction. I do list my best couple of fiction sales, but try to list ones in the same genre as the magazine I'm submitting to.

Yes, I do, but I sort of summarize them by saying, I've had (number) stories published in (magazine name), etc. I don't list them by title. I also list any anthologies my stories have appeared in, but just in general.

It's called my writing biography.

If it's a paying market, I would mention it. Even if it was from ten years ago (though I wouldn't mention that it was ten years ago). The point is that you were published; that doesn't lose its merit over time.

Many...

editors ask for a brief CV/bio with a submission.

Tailor your credits to fit the journal/zine/magazine, as people here have suggested. And if you are submitting to a literary journal or a general magazine which occasionally publishes fiction I would mention your top non-fiction awards/credits.

Honestly, I have about 3 different cover letters. One just thanks the editor for their time and doesn't include any publications or awards (even though I have that pro sale, etc.), the other lists a couple publications (last two-three) and the third mentions interseting things I've done (like worked in Bone Marrow Transplant) - to tailor to what the magazine asks for.

From what I understand (both from reading interviews and doing slush) cover letters don't tend to matter. I never read them as a slush reader anyway, many don't. It's the first page of that story that gets people to read the rest of it.