its very obvious the sex of adults - and when you have the eye for the first indications I think at like 2-3 weeks you can see if the fry is gonna turn male or not.
that 6 gallon tank could take a lot of endlers - they are hardy fish and are small... I have a lot in my tank - probably like 12 adults and a lot of fry in my 5gallon - I know that is a lot, I am about to sell some to some local club members however I change water usually 3 or even 4 times a week and they all appear healthy and happy to explore the tank interacting with each other
