At this time there are no known compatibility issues. However, it you wish to install El Capitan at your venue, we advise doing it to only ONE machine to start with and use it for a few days to find out if you have compatibility problems with other applications used in conjunction with Theatre Manager. We cannot say for sure if all your other important applications work, so best approach is one machine at a time.
Please do not update the postgres, apache, or second generation listeners servers at this time. Those servers also work with El Capitan - but save those for a couple of weeks.