kevy - I think what you've read is correct (IMO). It's up to you really, but I think if you have a large enough tank (which has to be pretty darned large), you could keep 6 happily. Otherwise you're going to need to stick with one to be safe.
According to aquariumfish.net (who's info I have come to respect) -
"When they [red tailed sharks] live together in large numbers in a large aquarium, they get along well with each other. They quarrel but almost never hurt one another.
But if you have less than six sharks in the same aquarium, they'll begin to quarrel, and the weakest ones will be miserable. So if you keep Red Tail Sharks, keep one or keep several, but don't keep just a few together. This guideline is true for most aggressive fish." (Also - this website said barbs can be a good tankmates for these sharks.)
I'm sure it depends on each shark, since all have a unique personality, and also on your housing for them. If it were me, I'd go for it if I had your setup - rts's get around 5'' and longer, but if you have 6 that's only 30'' of fish in your 55 gallon. Of course, it's going to depend on how many barbs you have. Overall, it's up to you, good luck!