Hi,
That message comes from WPFC, so please ask on their support forum, why is that message there.
My guess is that it’s because both WPFC and FVM have minification.
When you do minification on 2 plugins, you are doubling the work of the server and it will probably cause conflicts. But their plugin (and any other cache plugin) works well with FVM as long as you do the minification on FVM and disable it on their side.
However, a lot of people think that enabling both will make it smaller… and it doesn’t work like that. It will cause conflicts, so the easier solution is to recommend to “disable the other plugin”.
I’ve used WPFC in the past, but now I would recommend W3 Total cache instead, or WP Super Cache. I’m an advanced user, so for me, W3 Total cache wins due to all the options it has. Speed wise, there’s no difference between any of those.
When you purge FVM, “you must” purge all other caches, that’s why the plugin tries to do that for you.
If you were to skip purging the cache when you purge FVM, the css and js files that are referenced on your cached pages, would no longer be there, resulting on missing javascript and css files.
Purging all caches, after purging FVM is the intended behaviour.
That message is unrelated to that… it’s just a message they print whenever FVM is present, because they believe, FVM may cause some trouble for their plugin.
To me personally, I would use FVM and W3 Total cache, but any other cache plugin should work, as long as you don’t enable their merging and minification, on html, css and js.
My advice is to disable FVM first and try to check if you’re happy with it (because one plugin is better than two). If the speed is not satisfactory, then use FVM as you’re using it now.
