This is genius. I've enjoyed Radio Garden on occasion for the musical novelty but never thought about putting a random station in Latin America on to practice listening to different Spanish accents.
Of course the news on any station is going to use more formal language, and the entertainment programming even on the flagship is going to use less formal. Just, the talk station from a flagship broadcaster - like BBC Radio 4 - is going to have the most bang for your buck in language instruction.
Seems obvious, but bears mention. Enjoy!