I think its important to realize that the RA is not THAT huge. They rely on turning countries away from the WG to fight for themselves.
As for their Commanders, they have 6 in total (5 without Kuma), plus Sabo and Dragon. Effectively 8 total major fighters at least if Kuma comes back. Funny enough, there are at least 8 CP-0 members.
I've always wondered what was the true purpose behind CP-0, and im starting to think they were created to eventually serve as the major battles against the RA. Why? Because while CP-0 serves to protect the Tenryuubito, the RA seeks to take down those people specifically. They have the exact opposite goes, and I don't think that is coincidence.
Also feel that Luffy and crew directly fighting CP-0 members would just retread CP-9, so its better if the RA's handle them in the final war, while Luffy deals with Blackbeard, Shanks, Marines, WG (Im/Gorosei etc)