Only seven people this century have been killled by GWS, fewer than those have been killed by bee stings. -> Is it considered parallel?
Still wrong. You can't say "Things are fewer than other things", ever, about any things (or people or animals or whatever).
I don't have a solid explanation for you -- think of it as an idiom, if you want to -- but you can't.
In New York City, men are fewer than women. No.
In New York City, there are fewer men than women. Yes.
New York City has fewer men than women. Yes.
By the way, this will likely have zero importance on the GMAT, because, as stated above, all you need to do is look at the parallelism of the things in the comparison. But, if nothing else, it will help you become a better writer of English.
Most importantly, do not create your own versions of GMAC's sentences.
Don't do it.
Ever.
It's a complete waste of your time.
In 99% of cases when students "tweak" GMAC's sentences, their versions are wrong, for reasons that GMAC does not test. (The range of things tested on SC is a very, very small fraction of all issues in English.)
So, we end up with a situation that's like "No, that's wrong, but you don't have to know why."
Like this one.
Don't do it.