What is the tank size?
Male Endlers are somewhat aggressive,but not to the point to where it becomes The Hunger Games.Depending on your experience,I would say keep about 40-50;with a lot of water changes that is.You could probably sell the rest to a LFS for about fifty...
Figure out the sexes first.If you have the 1:2 ratio,you should be fine.If you don't,with decent filtration and water changes you should be able to add two more of whatever you need.
Poor water quality could've also been a culprit.
He is most likely stressed out.I would imagine you don't have a test kit,If you do,test the water for Ammonia,Nitrite and Nitrates.If you don't,do a 50-75% water change.
Sorry,even if you do have a test kit,do a water change
I would say yes.The same thing happened to me,I got a Mickey Mouse Platy and she gave birth to Mickey Mouses even though there were about 5 other species of Platies in the same tank.Like @Floundering_Around said,if you want to keep the strain,get another one just to be safe.