As far as bioload is concerned 6 endlers will not be an issue, especially since it appears you're running a canister. It becomes an issue of how many you can ethically stuff in there, I personally have 8 endlers and a bunch of shrimp in my heavily planted 5.5 gallon, all of them are happy and healthy with no aggression between any of the inhabitants so in my opinion you'll be just fine with 6 endlers.
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