From your other post, this is a 96 ltr tank=25 US gallons, so corydoras would indeed be a good option. However, it is also a new tank, which is still going through the cycling process, so 4 platies is really all you want to keep for the next month or so. I would strongly advise you to spend money on a test kit, so you can monitor levels of ammonia and nitrites. Read the pinned topics on cycling and you will see what I'm on about. Once your tank is cycled- that is once there is no sign of either ammonia or nitrites even a week after a water change- you can slowly add more fish.
"Cleaning fish" may be a good option- simply because some of them are very charming fish, but you should not expect any fish to do all your maintenance work for you, and even "cleaning fish" need to be fed their own food. Some common cleaning fish include:
corydoras- these are peaceful schooling fish (so need to be kept in groups), which eat food that falls to the bottom, but not algae. They must be not be kept on a dirty substrate though, with old uneaten food, as this will make them ill. So you still need to gravel vac. IMO these are some of the most charming fish in the trade
a bristlenose plec- eats algae and fallen food, but produces a fair amount of poop, so you will still need to gravel vac, again peaceful (thought can be territorial amongst themselves) and quite happy to be kept on its own
otocinclus- small algae eaters that prefer living in small groups, very efficient but sensitive to new tanks
shrimps- eat algae and fallen food, like to be kept in groups, low bioload
All the above will get on well with your platies. Three fish to avoid are the common plec (aka sucking loach), the sailfin plec (d

) and the Chinese Algae Eater (aka sucking loach/golden sucking loach). These will outgrow your tank and/or spell trouble.