Should I write a paper describing my comparisons and user guides for free tools?
The exploratory work that you have done in non-functional testing tools can be easily summarized in a kind of survey paper. You need to enumerate the tools that you have played with, describe the features and limitations (even with a summary table) and describe your tool selection process (free tools seems like a criteria that you did taken into account). Further, you can even detail the tests that you did with the different tools.

You can then make a extended critique over the existing tools, pointing out the most valuable features and, then, pointing out the lacks, limitations and suggesting further research directions that need to be done in that area (e.g. suggesting the implementation of missing features or other ways of testing).