There aren't many tanks that can house a full grown common pleco either. A pond really is the only reasonable solution for someone who wants to keep them, and not a tiddly little pond either.
OP, consider returning or rehoming the Chinese Agae eater too. As you've discovered, they're fine when small, but as they grow they get more territorial and nasty, and the other fish will pay for it. Same with a red-tailed shark, which get far too big for a 20g, and will make your other fish miserable in the 55g.
I'm sorry you've been mis-sold fish, and had such a rough introduction to the hobby! It's admirable that you got a much bigger tank to try to house them all, and I understand that you've named them and got attached. But as a pet owner of any kind knows, you have to be realistic about whether you can provide them with what they really need, and it's really not possible to house a common pleco, a red-tailed shark and a Chinese algae eater happily for their lifespans in a 55g and a 20g, let alone with the other fish you have. In these cases, rehoming, or returning so the stores have to face rehoming these fish that they sell as cute little juveniles, not as the tank busters they become - is the right thing to do, for the fish as well as yourself.
For future refence, and so you can figure out how to provide the best for your other fish, a reliable source is Seriously Fish. They have detailed profiles on most any of the species of fish you're likely to encounter, and you can trust the info given there. There's a lot of misinformation online and from the stores, as you've found, but if you look up your current and any potential future fish *before you buy them!!* you'll be in a much better place.
www.seriouslyfish.com