If the goal is to disable all styles with the exception of those used by fancyBox, that should be accomplished by setting the Theme value to none, unless we missed something in testing. There is a master override under the Compatibility tab which disables all styles regardless of theme chosen but it may not work for you either though it might be worth a shot.
At this point we are not clear regarding what the gallery should look like so it is difficult to tell which RPS Image Gallery styles are not desirable. If you get a chance, please provide a screen capture of the gallery with RPS Image Gallery disabled and one with it enabled.
Also, if you would like us to have a look at the theme you are working on, you can do so by uploading an archive and sending us the link to it using the contact form on our site.