This paper proposes, first, an analysis of ethnic and religious clasificactions of indigenous populations produced by colonial documents (Jesuit missionaries’ chronics and texts of Borbonic offitials) under the indigenous policy of reducciones in Paraguay and the Rio de la Plata. Second, an exploration of concrete modes of interaction between “reduced indians” and “infidel indians”, reconstructing specific contexts in which they took place. It is suggested that the official discourse reflected in the chronics and later texts was oriented to produce a fiction of order (and of disorder) based on the ideal of ethnic and linguistic homogeneity, and a radical antinomy that opposed Christian and infidel space. Said “fictions” do not allow an understanding of daily interactions of the mentioned populations in all their complexity, which requires the examination of local sources in order to reconstruct indigenous agency and social networks that go beyond interethnic relations and cultural belonging.