I have some cories, otos and 6 male endlers in my 10 gallon. I love the endlers but I think I would do male guppies if I were to do it over. The endlers are so tiny, often its like the tank has no occupants. Mind you my tank is heavily planted so there are lots of hiding places.
I just think guppies are extremely beautiful and larger than the endlers.
Or else I would do half a dozen of your fav tetra.