Dont get nerite snails they only eat algae and their shells crack and rot away unless the ph level is higher than normal for most tanks.
Unless you have real plants dont get it. Zebra snail is the same thing as a nerite just prettier color but its also harder to take care of and is smaller (nickel size)
You might be able to do 3 cory's if you do a lot of ground cover// shade from plants. Like you can get a piece of driftwood or decoration that takes up a lot of space and has lots of tunneling areas and also add lots of fake///live plants.
You can get a away with a few more fish in your tank if your tank is LIVE planted if bio-load is a problem.
Corys won't work because 3 of them need AT LEAST 10 gallons. Its not a bioload issue its a foot print issue. Nerite snails are fine.. just add a little cuttlefish bone or cleaned eggshells to the filter and you get enough calcium to keep the snails happy.
Corys are the ones that usually will eat nothing but actual algae.. they've been known to starve to death because they refuse to eat wafers. Nerite snails and mystery snails have no problems munching on wafers or algae or lettuce or uneaten fish food.. basically anything you want to give them.
*Be smart.. Do your research!!*[/COLOR][/FONT]