I don’t know about the bird ranking after 20 years, but you are totally wrong about the Ready Reserve:
http://usmilitary.about.com/od/airforce/l/blafrc.htm
Reserve Categories:
Ready Reserve. The Ready Reserve is made up of about 132,574 trained reservists who may be recalled to active duty to augment active forces in time of war or national emergency. Of this number, 70,118 reservists are members of the Selected Reserve who train regularly and are paid for their participation in unit or individual programs. These reservists are combat ready and can deploy to anywhere in the world in 72 hours.
“Additionally, 62,456 are part of the Individual Ready Reserve. Members of the IRR continue to have a service obligation, but do not train and are not paid. They are subject to recall if needed.”
No. You cannot both retire and be a member of the IRR. You may leave active duty and become a member of the IRR. However, law then requires that you serve at least six years in the Reserves to be eligible for retirement as a Reservist. Reserve pensions are approx. 1/3 of pensions for people leaving active duty. No one would make such a decision.