[@PigeonOfAstora]: The knights have to have been knighted(so presumably it's pretty difficult for a criminal or convict to get that sort of honor) and have a genuine dedication to the purpose of the Iron Rose Knights.

Technically, there are non-knight footmen and stuff there too(they're just not really counted as the Iron Rose Knights on account of not being knights), but presumably on the off-chance they were once a criminal they're likely very heavily reformed.