My apologies if this seems rude ghostknife, but I would disagree with you about the "algae eater" you are referring to. (which I am interpreting as a CAE) If it is a CAE you are referring to, I would strongly recommend chanman stay away from them. They will not stop at 2-3" but rather grow to 5-6" in length, and can be quite aggressive with other fish. It may not seem like it, as most people assume they are exclusively herbivores, however, they are opportunistic omnivores. Whatever comes their way, they will eat it. Also as I mentioned above, they have an aggression issue, which really begins to unfold once they surpass the 3" mark.
Pictus catfish, though I don't believe you were recommending one, get FAR too big for even a 55 gallon tank, and are nocturnal carnivores. Anything that will fit in their mouth will be seen as a potential food source. I owned a pictus catfish and an eclipse catfish a long time ago, and the two of them were eating a combination of shrimp pellets and feeder goldfish.
As far as an "all around clean up crew" for chanman, I'd go with a snail, or perhaps a few ghost shrimp. Snails do contribute to the bio load of the tank, as do ghost shrimp, however, they do seem to eat a lot of the leftovers in the tank. Snails are omnivorous, and will eat algae, scraps of food, even fresh vegetables you put in the tank for them. (remember not to leave the veggies in the tank overnight.. they can foul up your water pretty bad) Ghost shrimp are great for getting scraps of food that get past your fish, and in my experience will even eat some algae on decorations, gravel, etc. If you intend to have your platys breed, do keep in mind that not only will the parents and neons attempt to eat the fry, but the ghost shrimp will go for them too. One fun thing to do with ghost shrimp though is to feed them blood worms... its really fascinating to watch how aggressively they'll go after the worms and to watch the red color of the worms slowly pass through their bodies. That all being said, I want to emphasize that it is YOUR responsibility to keep the tank clean, NOT THE FISH. Fish which will eat algae and excess food are great for helping maintain the tank, but ultimately it is your responsibility to maintain a clean and healthy environment for your fish.
One other thing I'd like to ask: what is your water change schedule and filtration like chanman? This will really help determine what would be wise to consider adding to the tank. If you are only doing 10% weekly changes and have a 10 gallon top fin filter, don't add anything to the tank... you will only create a lot of frustration for yourself. If you are doing 25-50% water changes weekly or better, then you are taking good care of your tank and can probably support a few ghost shrimp or possibly a snail or two. Be careful of what kind of snails you get though... there are a few which can get as big as a baseball.
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