I had 2 male ruby reds in a 75 with 7-8 females for about a year w/out any problems. One male never showed full colors, then one day he started, and in less than 2 weeks one male had killed the other one.
The second eureka red male (in another 75) showed closer to full colors, but the dorsal and anal fins never grew as long as the dominant male's. That extra male was sold, as far as I know he's still alive and well.
You can keep an all-male tank, but it's best not to keep 2 of the same species or even different species that look too similar to avoid potentially fatal conflicts.