for instance, after Bajor joined the Federation, COLONEL Kira of the Bajoran Militia became CAPTAIN Kira of Starfleet
Or then COMMANDER Kira, assuming she were a Lt.Colonel. After all, that's the uniform she got to wear when undercover on Cardassia, and it would hardly make sense to force her to wear a uniform for lower rank than the one she held in Bajoran service.
Although of course a promotion would probably be due if Bajor joined the UFP, a development in which Kira would have played an active part...
...one of the (few) requirements for Federation membership is that a world MUST have a single global government.
What is a single global government? Does it suffice that it speak with one voice in matters of foreign policy? In that case, the planet might have plenty of nations in the current sense, as long as a unifying agency of some sort channeled the foreign policy decisions to interstellar space.
Some sort of fine structure for local representation will probably exist beneath the unified level anyway. Few UFP members are likely to be ruled by an absolute sovereign who allows for no dissent whatsoever.