I suspect that you are following a good lead that may definatively connect the 'Giusino of Sicily' with the 'Jusino of Puerto Rico', but I have not seen any further posts regarding the information you gleamed from your copies of the records from the 'Guia de Fuentes Documentales de Archivos'. My hope is that you could share your findings (names / dates / record reference) or at least provide the records' reference code, so that I might get a lead on researching those records myself.
The oldest record reference of a 'Jusino' I have come across is of one Ambrosio Jusino, who brought, in 1659, a nonspecified demand action against Pablo de Lasa y Olivares, heard within the jurisdiction of José de Novoa y Moscoso, Governor of Puerto Rico, with the action reaching a nonspecified conclusion in 1665.~ Ref: 'Archivo General de Indias', Title: RESIDENCIAS PUERTO RICO reference code # ES.41091.AGI/16415.9.3
Other than that, I came across two later references of early Jusino settlers in PR:
One of another Ambrosio Jusino being born in Cabo Rojo, Puerto Rico, in 1765.
And of one Patrizio Jusino recorded as a landowner in Sabana Grande, Puerto Rico, in 1787.