So the answer is that Starfleet couldn't forcibly remove either colony, if they renounce their Federation citizenship, but if they are considered citizens, then they are able to be forced out by the Feds
No, what allowed the Federation not to remove the colonists in Journey's End wasn't their renouncing their citizenship, it was the Cardassians permitting them to stay, despite the conditions of the treaty between the Federation and the Cardassians.
If the colonists had still renounce their citizenship's, but the Cardassians still insisted upon the letter of the treaty, then Starfleet still would have ultimately removed them.
In the case of the Tau Cygna V colonists, it wasn't my impression that Starfleet was ever going to force
them to leave. Picard apparently expect them to voluntarily leave once they understood the situation. In the end that's what happen.
Given the numbers of the colony, and the length of the Shelak time extension, if the colonists hadn't decided to willingly go I don't think there would have been much the Enterprise could have done about it.