You can use the Cloudflare with WP Fastest Cache. In order to minimize potential conflicts, you need to disable the optimization options of the Cloudflare.
1. Disable Auto Minify
You should disable this option because sometimes it breaks the site. In addition, these options do not help for optimization because WP Fastest Cache increases the result enough.
2. Disable Rocket Loader
You should disable this option because sometimes it breaks the site.
Don’t Use CDN Integration
If you use Cloudflare, all the sources of your site are already served via Cloudflare so you don’t need to use CDN integration for Cloudflare. However, if you want to use any other CDN system such as Maxcdn etc., you can use the CDN integration.