The Kazon aren't from either the Alpha or Beta Quadrant, they're from the Delta Quadrant.
On the bright side I've heard that when STO starts focusing on the Delta Quadrant the Klingons are going to get Kazon and Starfleet the Ocompa. Talaxians will be a playable race as well, but I don't know for who, maybe both. If the Romulans are a faction by that time, they'll probably get the hurogen.
I thought the Hirogen were already planned as being part of the RSE from the beginning (meaning whenever the faction is created). There is already an in-game reason for them to be there. Sela made contact with them in exile and brought several ships/fleets over when she overthrew Taris.