The Typhon Pact series revealed that the Breen are four different species (at least) with the Fenrisal, Paclu, Amoniri, and Silwan being identified. With the fact that at least twice in the DS9 series the main characters had commandeered Breen armor (presumably either stealing suits left lying around or killing their owners - i.e. Kira and Dukat in Indiscretions and Kira in What You Leave Behind) I wonder how it could be conceivable that the fact that the Breen are at least four different species remained unknown till Bashir's undercover mission in the Typhon Pact. I'm thinking one of two theories came about. Either the two they killed were Amoniri (the sort that evaporate when exposed to natural air) or the latter theory: Given the Breen are secretive I'm certain that bodies of their soldiers are fitted with some form of disintegration mechanism (minus the aforementioned Amoniri who evaporate if exposed to normal air anyway) to destroy their bodies to prevent hostile forces from learning anything about their physiology.