The Hirogen existing are just the two unique doffs. There were already two Vorta and two Jem'hadar when the gamma quadrant pack was added. For that matter, the Xindi already in the game don't preclude more common Xindi, since originally there was only one ultra-rare Ferasan and they became the KDF counterpart for Bajorans.
As for the Kazon, there's already a lot of doff races that don't play well with others, but specific to the Kazon it's the gang-like nature of their political system that keeps them from working with each other. Removed from their leadership and placed into a different command structure there's no reason to expect their politics to come with them. They'd work especially well as a KDF race as they appear to respond fairly well to an authoritarian system.