I am referring specifically to Gambusia affinis . There are a number of subspecies. G. affinis and similar species are insignificant in color, a melange of brown, dull yellow, and black. Females grow up to 3.25 inches while males grow up to 1.75 inches depending on the subspecies. Usually the fish stay around an inch long.
Mosquito fish are far from perfectly peaceful.
Mine used to harass each other and other fish. In a pond setting, they are less bullying. They also will eat some of their babies. Each fish should have a few gallons to itself but can thrive in high densities with more than five fish per gallon. They will live at least a few years.