Hi, first of all, make sure that you planned migration from CentOS 8 to a supported Linux version due to the fact that
CentOS 8 reached EOL
There is no generic recommendations for CloudFlare(CF) rules and Plesk.
CloudFlare filters HTTP, HTTPS traffic, therefore if you have DNS configured on CloudFlare side, make sure that you disabled CF rules for MX records, etc.
If you are going to use WordPress CMS, there is a probability that CF plugin for WordPress will be necessary - check
this.
Another possible issue that can be caused by CF with issuing Let's Encrypt certificate - check
this.
Since that Plesk and CloudFlare constantly improve their product and services, there is always a probability of any integration issues. And if you have some particular error or question do not hesitate to ask us here or on a support portal.