I have never heard of anyone grafting feijoa onto guava rootstock.
The most frequently eaten species, and the one often simply referred to as "the guava", is the Apple Guava (Psidium guajava). The genera Accara and Feijoa (= Acca, Pineapple Guava) were formerly included in Psidium.
Maybe give it a try and report the results.
Ed