Psuedotropheous saulosi are also an option. They are easily sexable, males being bright blue with black, females are pure golden yellow. They stay smaller, at around 3.5-4 inches, and IME, aren't that aggressive. I currently have 7 of them, 3 males, 4 females, in a 40 tall with a gold nugget pleco, an albino bristlenose, and a male M. johnjoansonae.