To save file system spaces, is it safe to remove "non-mst" files that are older than certain number of days?
Yes, if you are not interested in any old or past SLA data to debug. The data stored in the "non-mst" files is related to forecasted data relevant to the SLA deadline. The data from these files is not used for any reports but it is used to display the table/time views in the Web UI when you select that old SLA deadline, if that deadline is still visible in your Web UI. This is dependent upon the online SLA count value.
To be safe, you may take a backup of the files to be deleted by creating a tar file and moving it to a different machine or keeping it local.
SLA trend reports are derived off of the mst files in both the final and archive directories. If you do not care for older SLA details that are in other files, then you can keep just the mst files and delete the other files.
The files from final directory are moved to archive directory based upon the SLA online count value defined in Admin Tool - SLA -Forecast panel. You can refer to the help dialog in Admin Tool or review online documentation on how to Configure Forecast Parameters.