If you wanted to keep just demasoni you could do it. There is enough bottom area (with the 18" width) to set up the required rock-work for this species. From lot of reading this species seems to do better in groups of 12 or more.
But they have a reputation for being agressive and numbers declining because they kill each other. My feeling is that in a large group with too many males, the dominant male will kill other males, a problem in a small tank.
For you to be succesful my approach would be get 20 juveniles and gradually remove the "picked on" (possibly subdominant male) fish as they grew. Hopefully this would thin you down to a nice stable colony. You really could only have 1 male in the tank! (Difficult to achieve when the males and females are virtually identical. You would have to be prepared to be vigilant. Having said that - Proceed with caution.