I'm always freaked out by all these stories of females in sororities coming down with columnaris. I've always wondered whether the underlying stress of living together is what causes these outbreaks to occur, or whether it has something to do with the 'overstocking' to minimize aggression that is generally recommended.
I would say it's unrelated to stress. All my girls came from either walmart or petco (sorry but im not shipping a fish from Thailand to Alaska), different ages, different sizes. I had some girls that when they were in a smaller tank alone they were an ugly olive color and stress striped. Nothing I did seemed to fix it till I started a girls tank. After a few days in it, the stripes were gone and they colored up to be some of the most beautiful fish I have seen.
I highly recommned having meds for such things like velvet, ick and columnaris on hand before setting up a sorority. I dont have access to alot of the meds locally and by the time they would arrive via amazon.com, the fish will be long gone.