Hehe, probably my favorite fish! I used to study them in my aquariums, etc. still writing an article about them...
They don't have to be in schools to really thrive, but it does help out some. Of course, if you have the room and money, the more the better. Like said, they're more of a freshwater fish- but can go up to higher salinities. In the wild, they're known for swimming between different sg levels throughout different waterways (sometimes even being seen in close to saltwater like conditions).
And yes, they are prone to fungus- i've had around 3 die from it, jeez, even my 5 year old one did! 5 year old dominant male, almost went through with breeding around 4 times, and ended up dying to some simple disease like that.
I always hear of how hard it is with feeding, but in my experience, everyone i've had converted within a day to 2 weeks. Starving periods on a few of them worked, but mainly, just throw freeze-dried krill in during feeding and it will happen. Having one that eats, then adding others, will help out greatly. I purchased a small one to go along with my male- saw it was eating krill, ignored the feeders and ate shrimp, krill, etc. in the first 2 days. Get a few rosey reds on hand, quarantine, treat, and feed them good. Have a stock on hand for up to two weeks, your going to want to gut load them when they first get in, then try fazing in "dead foods"- of course, only after you've tried the dead food in the first place. I also suggest treating them internally for parasites, most are eating feeder fish when they get to you- and i've had a confirmed case of one having flukes attached between some flesh and the air bladder.
HTH, anymore questions?