I am always happy to answer reader mail and over the years I have answered a lot. I'm not exactly the fastest responder, but I do my best to answer them all. On occasion, I get repeats of the same question so I thought it would be good to answer some of those publicly to help out other readers who may be wondering the same thing.
Both of these pens use what is called a D1 size refill, so yes, the Slims refill fits fine in the Sharbo X. Most D1 refills are interchangeable between brands, although the small tolerances might make the fit tighter or looser depending on the pen. For example, the Slims refill fits in the Sharbo X like a glove, but I had to force the Sharbo X refill into the Slims barrel. In the end it fit and worked fine, but some force was required to seat it properly.
I hope this helps you out Mike, and hopefully this helps other readers who had the same question.