Good afternoon,
Cloudflare's nameservers are authoritative, while Google DNS and OpenDNS are recursive. If you are curious to know more about this subject, you can follow this link: what is DNS.
Cloudflare's DNS delivers superior performance, high availability, and robust security. For more details, see here: Cloudflare vs Google DNS.
Yes, you can mantain your host's DNS and delegate to Cloudflare nameservers, although conditions apply (please check custom nameservers).
Thought process: Assuming the client has either a Business or Enterprise plan, I answered with references to the respective pages with more information.