It would be fine for Endlers, a good size for an Endler only tank in fact!
I probably wouldn't go with cories in this tank. The cory species that could tolerate water that's hard enough for Endlers would get a bit too big to have a school of six or so in that size tank, and cories do better in the right school number, they're such a sociable little fish.
I do have six sterbai in 60 L tank right now though, as I type this. But only because I know they'll be upgrading to a larger tank soon, they're not fully grown yet. It would be too small for them long term I think.
Otos and pygmy cories are out since they need softer water than endlers too.
How about shrimp? You could have a beautiful colony of neocaridina davidii in there, and they come in a variety of colours. They do a good job of picking the tank clean of uneaten fish food and algae, entertaining to watch and breed well too. Red cherries most common and usually the cheapest/easiest to get hold of, but can also get different blues, yellows, or even rili.
People say not to mix the colours, because after a few generations of breeding, you lose the main colour and they tend to revert to the brown wild type. And that's true, especially if you plan to sell any young and want to keep them a high grade colour. But I quite enjoy spotting the different colours mine have thrown by mixing just a couple of blues in with the red cherries. Yes, get some brown wild types, but some of those are cool looking too, and they've also produced blues, blacks, oranges, even one perfect rili one time. So if it's just for fun and your own personal colony and not for profit/high grade shrimp, I say put whatever colours you want in there and enjoy.
