I'm sorry to hear about your depressing experience
I also would like to ask how you cycled your tank? You mentioned a lot of dieases; you probably don't have all the ones mentioned but you may have some of them, but I don't know whether we'll be able to determine which one(s) any better than your own diagnoses.
4 bala sharks and 6 barbs is a lot for 55gal (well, depends on the size of 4 bala sharks) - if your tank was improperly cycled, it could have been quite a large bioload to introduce into an unprepared tank, and all the problems could have been cause / exacerbated by ammonia burn / new tank syndrome.
I noticed you said you planned on keeping your bala sharks in your 55 gal until adulthood. You won't be able to do that. Bala sharks are BIG, BIG fish and with 4 of them you should be planning for at least 125 gallons.
As rba said, quarantining is very important (a lot of us learn the hard way
.) I do also suggest not to rely entirely on Petsmart for fish advice. Sounds you bought a lot of meds there (which is what any petsmart employee will encourage - but is not always necessary.) I personally don't like medicating my fish unless it is necessary, as the meds are hard on the fish and the bacteria in your tank and can do harm if improperly applied or dosed.
Anyway, as you say, now is the chance for you to determine the cause of the problems you've been having.
Have you tested your water parameters? What are they?
Anyway, for the time being, keep the temperature a little high (81) and keep the water well aerated with an airstone.