With the new load of fish your ammonia spiked and basically poisoned your tank. Since the Feeders you picked up are kind a used to "BAD" conditions they are a bit more tollerant. The Algea Eater (from your discription) sounds like a Plecostomus which needs a heater.
As mentioned all your fish will eventually out grow the tank (if they live to maturity). A dozen two inch fish in a 55gallon isn't over stocked (yet).
As for explanation on your fish you'll have to look and read up on that.
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