Many years ago I had some nasty bouts of columnaris, but it was before the maracyn's were carried at my LFS. I treated them all with Jungle Fungus Eliminator/Cure, and it worked really well.
As for how to treat, I'd definitely take her out of the sorority (and away from the plants, which would probably definitely not benefit from the meds), and treat her in quarantine.
As for the others, I wouldn't treat them unless they show signs of disease. Most diseases are opportunistic, and will only hit fish who are weakened from something else. (and it can be anything, stress, spawning, temp changes, water quality changes (good or bad, it's still stressful) the list goes on and on) But, make sure you watch the others carefully, and if they show signs of disease, scoop them out and into QT as well for treatment. :) I can't remember the ingredients for maracyn II, but here are the ones for the Jungle Fungus Eliminator (which I still have on hand from forever and a day ago...I'm sure it's expired, but I'm a hoarder... ;): sodium chloride (salt), nitrofurazone (antibiotic), furazolidone (antibiotic), potassium dichromate (chemical that helps with parasites and fungus). I believe that's very similar to one of the maracyns, but I'm not sure which. :)
I hope that helps!!